I have been looking through all the different armies in 40K and here is my take on the top Relics in the game right now, the ones a competitive list should not leave home without (in no particular order):
 GORGON’S CHAIN (Iron Hands) — Gives Bearer a 3++, Eternal Warrior, and +1 to his FNP (does not replace a weapon); as the Bearer loses wounds, this item loses some of its benefits (but regains them it the Bearer regains the wounds somehow).
*Rationale: Not only is this one of the great durability enhancing Relics in the game (especially when combined with something like a Narthecium from an attached Command Squad or the Endurance power) but, unlike the Shield Eternal, you do not lose out on the ability to fight with two close combat weapons for +1A when using it. The classic way to use it is with an Iron Hands Chapter Master… give him a bike, Artificer Armor, PF, LC, and the Gorgon’s Chain and attach him to a Biker Command squad and you have a character that is 4W, T5, 2+/3++, with IWND and FNP(3+)… on top of this, he puts out either 6 PF or 6 LC attacks on the charge at WS6, so a true close combat beast on almost every level.
 HUNTER’S EYE (White Scars) / MULTI-SPECTRUM SENSOR SUITE (Tau) — Both of these relics do exactly the same thing, which is allow the shooting of their unit to Ignore Cover, with no test of any kind required (the Tau MSS only works if its bearer does not shoot himself, while the Hunter’s Eye still allows its bearer to shoot and benefit from Ignore Cover).
*Rationale: I don’t think I need to explain how powerful these two Relics are, especially when combined with a powerful shooting unit, since Cover Saves are one of the key forms of survivability for everything from hordes of light infantry to Jinking Bikes/Skimmers/FMCs to even vehicles using Smoke Grenade Launchers. Being able to remove this protection with 100% reliability is amazing and can be a game-changer in almost all circumstances. Some of the best uses for these Relics include attaching their bearer to Grav-Cannon Centurions, Missile-side Teams, Long Fangs with Plasma Cannons behind a Haemotrope Reactor, and Deep Striking Crises Suit Teams (with almost any weapons configuration), though the possibilities really are almost endless. Imperium forces probably get the most out of an attached White Scars IC with the Hunter’s Eye, since there are no limitations to its abilities and since Tau already have easy access to Ignore Cover via Markerlights, but these two Relics are definitely in the “don’t leave home without them” category.
 GRIMOIRE OF TRUE NAMES (Chaos Daemons) — Roll a dice, on a 3+ you cause the target friendly Daemon unit to improve its Invul save by +2, on a 1 or 2 roll, then they make it worse by -1.
*Rationale: Another of the most “famous” Relics (since several Chaos Deathstars are built around it), in conjunction with the Cursed Earth power (from Malefic Daemonology) you can bring a selected Daemon unit up to a 2++ (which becomes re-rollable if the are a Daemon of Tzeentch). Combine this with a powerhouse unit, like a Daemon Prince/Daemonic MC, a horde of Flesh Hounds, or even allied CSM Daemons (like Possessed, Obliterators, and Warp Talons) and you have a durable, mobile, and deadly unit which cannot be ignored, but also can shrug off entire armies worth of combat power. Of course, there is some risk associated with it, but that is why so many Daemon players us Fateweaver for the re-roll to the Grimoire (i.e. it goes from a 66% chance of success to an 88% chance with Fateweaver support).
 TALISMAN OF BURNING BLOOD (World Eaters) — Add 3″ to the Movement, Run, and Charge distances of the Bearer and his unit.
*Rationale: Another very powerful relic that you can build an entire Khorne army around. One of the best builds I can think of for this is a unit of World Eater Bikers from the Butcherhorde detachment, along with up to five attached Chaos Lords on Juggernauts… give the Talisman of Burning Blood to one of the Lords and outfit the rest with close combat Relics and you have a unit that is T5 across the board, hits like a freight train thanks to Rage + Furious Charge + Khorne Relics (and can even split into six different units to charge multiple elements of your opponent’s army), and also has ridiculous mobility. How much mobility? Well, this unit gets a free 2D6″ movement before the game begins (which does not prevent charging on turn 1, unlike Scout), can move “normally” up to 15″, then charge another 2D6+3″ (with re-rolls for failed charges), which gives it an average Turn 1 charge threat radius of 32″… pretty impressive! As I mentioned, the whole unit can use this, then split up later turns to maximize the “choppiness,” and generally cause all kinds of problems for an opposing army, especially a shooting-based force like Tau or even Eldar.
 ASTRAL GRIMOIRE (Thousand Sons) — The bearer can make one friendly unit within 12″ (this can be his unit, if he wants) into Jump Infantry for the Movement Phase each turn, with no requirement of any kind of Psychic or Leadership test.
*Rationale: This is one of my favorite Relics, and one that I have already tried building an army around, because it is so reliable and literally doubles the mobility and flexibility of otherwise slow-moving units, be it Scarab Occult Terminators, Obliterators, 20 man blobs of Rubric Marines, Possessed, or even Tzaangor Warherds. Add in some Psychic support to make the unit even more durable and potent in shooting and/or close combat and the Astral Grimoire can really turn a good unit into an elite, “flagship” battle group for your whole army that will be a threat to all kinds of opponents. So far, my favorite build is 10 x Thousand Sons Terminators with Combi-Meltas, Chainfists, Reaper Autocannons, and an attached PML(3) Sorceror (he carries the Astral Grimoire for the unit)… if you get a blessing off on the unit, then the Terminators can move 12″ a turn, shoot either Bolters/Reaper Autocannons or Meltas/Reaper Autocannons, and charge in with their Chainfists, all while rocking 2+/3++ saves to make for a unit that can really do a mix of everything well.
OK, that’s it for now, will look at the remaining Top 10 later, plus some honorable mentions in there as well 😉