Continuing to look at what I consider the “top 10” Relics in 40K right now:
 KUROV’S AQUILA (Astra Militarum) — Taken by Company Command Squad Officer only, this gives all friendly AM units within 6″ Preferred Enemy and also allows the Bearer’s squad to re-roll a single failed Ld test per turn.
*Rationale: A very simple, but exceptionally powerful, Relic. There are so many application to being able to give out a Preferred Enemy “bubble” in your army, but some of my favorite include giving it to nearby Leman Russ Executioner squadrons (PE allows re-rolls for Get’s Hot and also make the Plasma Blasts wound on a re-rollable 2+ against most infantry), infantry blob squads (this works for both shooting and close combat, don’t forget), nearby Basilisk batteries (basically auto-wounding anything they hit), and even the Command Squad itself (i.e. jump out of a Chimera or Valkyrie and unleash 9 x S7 AP2 Plasma shots, all with Preferred Enemy). This is especially outstanding because it is not limited to just one unit, but applies to all units with even just a single model within 6″ of the Bearer…. entire Astra Militarum gun-lines can improve both their accuracy and damage output with shrewd positioning of squads around this Relic. Finally, that re-roll of an Ld test can apply to an Officer of Fleet in the Command Squad, making his Ld7 much more reliable when trying to improve your Reserves or deny your opponent’s Reserve rolls. While it is expensive points-wise (12 MBs), I think almost any AM commander would do well to find a way to bring Kurov’s Aquila in their army list.
 PURETIDE ENGRAM NEUROCHIP (Tau) – Gives Bearer the ability to choose one of the following abilities each turn: Tank Hunter, Monster Hunter, Stubborn, Furious Charge, and Counter-attack.
*Rationale: While Furious Charge and Counter-attack only apply to the model and are generally not that great in a Tau army (where you almost never want to be in combat), Tank Hunter, Monster Hunter, and Stubborn all impact the entire unit that the Bearer is attached to. Stubborn is the worst of these three, since it is mostly useful for the Assault Phase and, again, it is already a very bad thing for Tau if you are having to use it there. All that said, being able to give “free”/automatic Tank Hunter and Monster Hunter to any unit that an IC attaches to is exceptionally useful, especially in the current Meta, where MCs, GMCs, and Battle Companies with loads of free vehicles abound. One excellent application of this Relic is attaching a Buffmander (Commander with Puretide Engram Neurochip, MSS, and C2 Node) to a max squad of Sniper Drones with an Ethereal attached as well… this gives you 27 TLed BS5 Sniper Shots at 24”, all with Ignore Cover and access to Monster Hunter… if you target an enemy Wraithknight, that averages 26-27 hits, 7-8 AP2 Wounds, and 5-6 Unsaved Wounds after FNP rolls… in other words, Sniper Drones with PENchip, Ethereal, and C2 Node support can straight up kill a Wraithknight in a single shooting phase, which is quite impressive! Alternately, if they can engage Magus the Red before goes into Swooping mode, they average 6-7 Unsaved Wounds on him a shooting phase (and that is accounting for a 3++ save for him), which should almost kill him or outright destroy him in a single volley. Against vehicles, combing the PENchip with Crises Suit Teams, Broadsides, or even Breacher Squads can dramatically improve their performance against armored targets. IMHO one of the best options is a 5 man Crises Suit Team with dual Missile Pods and Velocity Trackers and an attached Buffmander… this unit puts out 20 x S7 AP4 shots at 36”, all with built-in Ignore Cover, Twin-linking, and Tank Hunter… even against AV13 armor, that is an average of 4-5 stripped HPs a turn, without requiring any Marker Light support at all, and it can cause this same damage to AV13 Flyers as well, thanks to the Velocity Trackers on the Crises Suits.
 MEGA FORCE FIELD (Waaagh Ghazkull) – All models within 6” of the bearer get a 4++ save against Shooting attacks (if embarked on a vehicle, then only the Bearer’s vehicle gets a 4++, and not any nearby units).
*Rationale: Given Orks overall fragility to shooting (thank you T-Shirt saves), both in terms of troops and vehicles, the “normal” Kustom Force Fields carried by Big Meks are quite a valuable asset to any Ork army, since with savvy positioning of units you can reduce incoming damage by an average of 33%. Well, with the Mega Force Field Relic, you reduce enemy damage by an average of 50%, which obviously is a major force-multiplier for the army. While 6” makes for a somewhat limited “bubble” for this ability (and it only applies to MODELs, not entire units), it still can be a game-changer for everything from Trukk lists to Mek Gunz to War Bikerz to even foot slogging hordes, if you place the Mega Force Field Bearer in the right location. I think this works especially well with, say, a Big Mek on a Warbike riding behind multiple Battlewagons… you can probably fit 3-4 Battlewagons inside the Mega Force Field 6” radius, if they are packed tight together, and now you suddenly have multiple AV14 vehicles that shrug off 50% of the penetrating/glancing hits from shooting attacks against them, which in turn dramatically increases their chances of delivering their passengers into combat, which is where all Orks want to be of course! Another effective use of this Relic is attaching it to a group of Nob Bikerz and using it to help them get across the battlefield safely, especially since there are multiple threats these days that can ignore the Nob Bikerz’ Jink saves (i.e. Wyverns, Tau units with MMS Relic, Imperium units with Hunter’s Eye Relic, Leman Russ Exterminators, any Tau units with ML support, any units with the Perfect Timing and/or Phase Form power cast on them, Necron Tomb Blades with the right upgrades, Template weapons of all kinds, AM units with the Fire on my Target! Order, etc. etc.)
 MASK OF SECRETS (Harlequins) / ARMOUR OF MISERY (Dark Eldar) – The Armor of Misery reduces the leadership of enemy units within 6” by -2 and also gives the Bearer a 4+/6++, which is a boost for most DE characters. The Mask of Secrets makes the Shadowseer carrying it Fearless (only Shadowseers can take it) and reduces the leadership of all enemy units within 12” of the Bearer by -2 (Note: Nothing prevents the effects of these two Relics from stacking for -4 Ld).
*Rationale: The uses of these two Relics are myriad, whether used separately or in conjunction with Dark Eldar/Harlequin allies. They can improve the chance of breaking enemy units in close combat, they can allow you to inflict lots of wounds with Dark Eldar Phantasm Grenade Launchers, and they make all kinds of Psychic shenanigans possible. I think one of the best applications for these Relics is as follows: take a Succubus with WWP/Armor of Misery and Shadowseer with Mask of Secrets. Use the WWP to Deep Strike them both with no scatter 1″ away from the enemy model/unit that you want to “assassinate,” in this case, let’s say it’s a Riptide. You subtract -2Ld for the Armor of Misery and -2Ld for the Mask of Secrets, making the Riptide/Drones Ld5. You then either use the “Mirror of Minds” power from the Shadowseer or, if you did not roll that up, use Psychic Shriek (which is guaranteed, as a Primaris power). If you have and successfully cast Mirror of Minds successfully, then the Riptide is basically guaranteed to die, since the Riptide has Ld5 and, adding a D6, the highest it can get is a “11,” while the Shadowseer has Ld10 and adding a D6, the lowest it can get is an “11.” This means that the Shadowseer will always tie or be higher and so will keep inflicting wounds on the Riptide until it dies, since there is no way to stop the “chain” by having the Riptide get a higher “sum” than Shadowseer. If you do nor roll up Mirror of Minds, then at least you can use Psychic Shriek, which with 3D6 gets an average roll of 11. Subtract Ld5 from this and you have automatically inflicted 6 wounds on the Riptide with Ignore Armor and Cover (so only Invuls and FNP, if it has Stimulant Injectors, can protect it). I think using Psychic Shriek like this can work well on other MCs or even on dangerous enemy units like Wulfen, Grav Centurions, Dreadknights, etc. and remove a lynchpin unit in an enemy army with high reliability.
 MOMENTO-MORTISPEX (Belisarius Cawl unlocks for Imperium Armies) — At the start of each of your turns, choose one of the following rules to apply to the Bearer and their unit (or weapons as appropriate): Cognis Weapons (BS2 Snapfire, Flamers do 3 hits rather than D3 with Wall of Death in overwatch), Monster Hunter, Skyfire or Tank Hunter.
*Rationale: This may be my favorite Relic in the entire game, just for the sheer diversity it provides and how it plugs a kinds of gaps in an arm without being super expensive (i.e. need some way to deal with Flyers/FMC, but don’t want unbalance your carefully crafted army list with otherwise one-dimensional AA Tanks… take this Relic; need to have a potent anti-tank unit, but you do not normally have access to Tank Hunters… again, take this Relic). Some of the best units to use this on include Grav Centurions, Astra Militarum blob squads, Kataphron Destroyers, Sternguard/Deathwatch Veterans, Deathwatch Terminators with CMLs, Grey Knight Purifiers with multiple Psycannons, Devastator/Long Fang squads, Gunslinger Vanguard Veterans, Sanguinary Guard with Inferno Pistols, Quad Mortar Batteries, AM Command Squads, Ravenwing Black Knights, and even on a Techpriest Dominus in a Cohort Cybernetica. For only 6 MBs, this Relic provides so much flexibility and is guaranteed to work every turn, with no leadership test or harnessing of Warp Charges required. Aside from the high “tax” of taking Belisarius Cawl to unlock it, I think this would be useful in pretty much every Imperial Army. (Note: I am not sure this is intended or that it will be legal after future FAQs, but right now it appears that an Imperial Knight which is a Character, like the High King in an Exalted Court, can take this Relic, which would make it IMMENSELY more powerful in a number of different applications… i.e. imagine Avenger Gatling Cannons, Rapid Fire Battle Cannons, and Stormspear Rocket Pods all firing with Tank Hunter, Monster Hunter, or Skyfire, for example).
Later I will post some “Second Team All-Pro/Honorable Mention” Relics that are still good, but not quite as universally amazing as these 10 that I have already looked at.