Looking at the rules that have just come out from the new CSM Codex, one thing that has jumped out at me in terms of competitive builds is Alpha Legion Infantry with the Mark of Slaanesh.
To start with, Alpha Legion have one of the best Legion Traits, “Hidden in Plain Sight,” which is an exact replica of Raven Guard Chapter Tactics and causes all enemy shooting at Alpha Legion infantry, bikes, and Helbrutes from more than 12″ to suffer a -1 to the to Hit rolls. This by itself already makes for excellent infantry units, especially if those units have good shooting at a range of 13″ or greater. On top of this, also like Raven Guard, the Alpha Legion infantry units get access to a unique stratagem which enables them to start the game in reserves and, before the beginning of the first player turn, set up anywhere on battlefield that is more than 9″ away from enemy units (so the “modern” version of the old Infilitrate USR)…. put another way, this basically gives Alpha Legion infantry “free drop pods” at the cost of a single CP per unit and also enables them to react effectively to how your opponent has deployed his forces. Taking these two Alpha Legion abilities together gives you infantry with significantly enhanced mobility and durability options right out the gate, before considering any other “force-multipliers.”
Now, what about the Mark of Slaanesh? Nominally, this does not do anything directly for CSM infantry units besides making them eligible to interact with certain rules, buffs, psychic powers, stratagems, Icons, etc. that are specific to the Mark of Slaanesh. Well, in the new CSM Codex there is Slaanesh-specific stratagem called ENDLESS CACOPHONY which costs 2 CPs and allows a Slaanesh infantry/biker unit to shoot twice in the chosen Shooting phase. There is also another new CSM stratagem called VETERANS OF THE LONG WAR (costs 1 CP) that grants the targeted CSM infantry/biker unit +1 to all its Wound rolls in either the shooting or fight phase. Finally, CSM Sorcerers using the Dark Hereticus discipline have access to Prescience (+1 to all Hit rolls for the targeted CSM unit) and Delightful Agonies (a Slaanesh-only power that grants the targeted CSM unit the ability to ignore Unsaved Wounds on a 5+).
So, you probably see where I am going with all this… deploy an Alpha Legion Infantry unit with the Mark of Slaanesh somewhere on the board greater than 12″ from enemy forces (deploy them via Deep Strike, transport, the Alpha Legion special stratagem, normally in your DZ, etc., etc., and ideally put them in cover for +1 to their armor save) and also deploy/Deep Strike a CSM Sorceror (equipped with JP or TDA) with Prescience and Delightful Agonies right next to them. On your first turn, cast Prescience and Delightful Agonies on the Alpha Legions Slaanesh infantry unit you have chosen, then in the shooting phase use both the VETERANS OF THE LONG WAR and ENDLESS CACAOPHONY stratagems on the squad. This gives you the following
1 – Shooting: +1 to Hit, +1 to Wound, and the ability to shoot their weapons twice that turn
2 – Survivability: -1 for the enemy to Hit them with shooting outside 12″, +1 to armor saves (if in cover), and the ability to ignore Wounds on a 5+
So, these are obviously very significant buffs, but how do they look when integrated with specific units? As I mentioned before, this entire tactic works best when used on CSM units with effective shooting at ranges 13″ or greater (so most Rapid Fire weapons are not effective in that range band), since that maximizes the powerful Legion Trait of causing enemy units to have a -1 to Hit when shooting at the squad. Considering this, the best three units that I can think of to employ with this tactic are as follows:
 “Multi-tool” Fire Support Obliterators — Now that in the new CSM Codex Obliterators get Assault 4 on their Fleshmetal Guns (it was Assault 2 before in Chaos Index), they truly are a more competitive unit and can really take advantage of all these buffs, especially since they come with the ability to Deep Strike (saving you from having to use a CP to “infiltrate” into position). While the S, AP, and D of their weapons are still random, with all these buffs they become very powerful indeed. Deep Strike the Obliterators in alongside the Sorcerer, hit them with all the psychic powers and stratagems described above and, on average, they put out 24 x S8 AP-2 D2 shots at a range of 24″. On top of this, they are hitting at effectively BS2+ and wounding with a +1 modifier (so, wounding on a 2+ against anything T7 or below, on a 3+ against T8, and on a 4+ against T9). Against, say, an Imperial Knight or a Wraithlord, they would inflict an average of 17-18 Unsaved Wounds in a single volley, denigrating its combat power with a lowered BS/WS and setting it up for other units to finish off shortly afterwards. Against a Land Raider they would inflict an average of 13-14 Unsaved Wounds, again, greatly reducing its proficiency and making it easy to finish off with the rest of your army. Finally, against T5 enemy infantry, say enemy Devastator Centurions, the Obliterators would inflict an average of 16-17 Unsaved Wounds, which would likely kill 4-5 Centurions in a single volley, in turn swinging the game significantly in favor of the CSM forces.
 “Tank-Busting” Havocs with Lascannons — Like with Obliterators, Havocs have access to solid weaponry that can effectively engage at stand-off distances, including Heavy Bolter, Autocannons, Missile Launchers, and Lascannons. While all of these weapons are great options with this tactic, I think Havocs with four Lascannons as a dedicated anti-tank/Monster unit is probably the best choice. With a nearby Chaos Lord (for re-rolling 1s to Hit) and 8 x S9 AP-3 D6 damage shots, against a T8 Land Raider the Havocs would average 7-8 hits, 6-7 wounds, 4-5 Unsaved Wounds, and 15-16 total damage, which would leave the Land Raider either very crippled in firepower/mobility and easy to finish off, or flat out destroyed. Similarly, against a T8 Wraithknight, the Havocs with all these buffs would average 7-8 hits, 6-7 wounds, 5-4 Unsaved Wounds, and 18-19 total damage, leaving it greatly reduced in combat power and with only a few remaining wound for the rest of your army to finish it off.
 “Dakka Dakka” Noise Marines with Sonic Blasters — On the other end of the spectrum from Lascannon Havocs, Noise Marines with Sonic Blasters can put out lots of anti-infantry, Ignore Cover shooting which this tactic can make even more efficient. 20 Noise Marines with Sonic Blasters and all of the buffs referenced above can put out 120 x S4 AP0 D1 shots a turn. With Prescience cast on them and VETERANS OF THE LONG WAR stratagem activated, against a horde of T4 Ork that averages 100 hits, 75 wounds, and 62-63 Unsaved Wounds, killing the entire mob many times over. With this much massed shooting, the Noise Marines can even take on tough targets, like a Tau Stormsurge LOW… against a Stormsurge, the Slaanesh Cult Troops average 100 hits, 50 wounds, and 16-17 Unsaved Wounds, bringing it down to BS6+ and making it “easy pickings” to finish off with the remainder of your army.
Aside from the very potent firepower that these three Alpha Legion Slaanesh infantry units can put out, thanks to their Legion Traits, cover, and the ENDLESS CACOPHONY power they should be able to absorb a whole lot of enemy firepower on their own. For example, if they are cover, then the Obliterators described above have effectively a 1+ armor save, a 5++ Invul, 3 wounds each, a 5+ ‘FNP-equivelant” save, and the reduction to enemy to Hit rolls by -1. In practical terms, if say a unit of 3 Centurions, each with dual Lascannons, shot at these Obliterators, they would get an average of 6 shots, 3 hits, 1-2 Unsaved Wounds (each inflicting D6 damage), 4-7 damage, and about 3-4 total wounds after the ENDLESS CACOPHONY 5+ saves… so, one dead Obliterator and one Obliterator with a single wound, which is a VERY economical trade considering what this unit would do to those same Centurions if they shot at them.
Similarly, Havocs or Noise Marines in cover (or behind a friendly Aegis Line) would have an effective 2+ armor save, a 5+ ‘FNP-equivelant” save, and the reduction to enemy to Hit rolls by -1. Given all this, a squad of 10 Hellblasters rapid firing their plasma incinerators at the Havocs/Noise Marines at 15″ away with 20 x S7 AP-4 D1 shots would average 10 hits, 6-7 wounds, 5-6 Unsaved Wounds, and 3-4 dead CSM models after the dust settles (thanks to the “5+ FNP” save from ENDLESS CACOPHONY )… that is A LOT of firepower that these squads could absorb and still be at well over 50% of their starting strength!