Originally Posted by
hyudun
(I'm assuming your wife hasn't played yet, or at least not extensively, and a medium point level, 1000-1500pts)
Plague marines I think are the easiest for beginners to get started with for CSM - they're incredibly durable, not to mention cheap since you only need 10 to have 2 squads and maximum special weapons. They're a very forgiving unit to play and morale-boosting for new players since they tend not to get shot off the board too easily. They also have the most fun default wargear with defensive grenades and poisoned melee weapons.
My next choice from a competitive angle would be Noise Marines, in large squads, with sonic weaponry, and an Icon of Excess. However, it's not too flexible/mobile and can be frustrating to play with for a beginner, despite being rather hard to kill and the potential to pump out a lot of shots. Not to mention it's rather expensive since you'll need the buy those upgrade packs.
If she's set on Slaanesh/Khorne and don't want to spend too much, then there are melee-oriented Noise Marines, CSM's with MoS, or any flavor of Khornite marines. Either way, they're all melee-oriented and I recommend a Landraider since they really need help getting to assault against pretty much any other army.
Thousand Sons may be a good choice if you are playing SM (you either playing them or IG, based on your comments) and your wife will be primarily playing against you. Like Plaguemarines, they're hard to shoot off (though PMs are harder, IMO), and giving her an army more suited to killing your units will hopefully mean less lop-sided battles initially as she learns and gets a better handle on the game.
After troop choices, I recommend:
A Chaos Lord on a bike accompanied by Chaos Spawn, so she's got something fast and maneuverable to play with
Obliterators - very flexible and can experiment with Deep-Striking & Reserves
Heldrake w/ Baleflamer - still a good unit despite the recent nerf, and gives a good understanding of Flyers and Reserves
Stay away from Warp Talons (don't do their job properly), Cultists (demoralizing to play with as a beginner, unless taken in Helcult formation or given a Fearless HQ), Mutilators (don't do their job properly unless in a Landraider), and Possessed (don't do their job properly unless playing as Crimson Slaughter).
Alternatively, you can just let her go unbound and grab whatever she thinks looks cool. There aren't any stand-out units for CSM other than Be'lakor and Nurgle Daemon Princes w/ wings and the Black Mace. Heldrake as a standout is debatable after recent nerfs.
Also, you're better off killing CSM's with Plasmaguns rather than Meltas. More shots/longer range.