Emperor's Children. I just love the myriad ways you can put them together;
1. Great Crusade/Pre-Heresy. The paint scheme is probably my favourite of any in 40 (alright, 30)K.
2. On the TT, you can use their higher I to make people cry in CC; or, you can load up on the Sonic Weapons (on the Cult troops at least) and make people cry when you shoot at them, although it is costly points-wise.
3. Great modelling opportunites. With their dedication to perfection, you can use art-y skills to freehand banners, NMM armour and really make the colours rich. You can use multiple kits to kitbash troops who embody the Crusade-era perfection, or the later corrupted dedication to excess, and everywhere in between.
4. If you can make a decent Fulgrim, I will love you forever! (And who would need anymore persuasion than that?

)
Beyond that, they all have such great background that you have lots of room to do whatever you want. Imagine the Legion of the Damned models as Night Lords, for example. Mix DC and Beserkers to give your army a really Khornate look.
I get all giddy just thinking about all of this! I've started my Chaos/Loyalist force which will be EC + WE using, primarily, the BA 'dex. I can paint up the other soon-to-be-traitor Legions to use as Tac squads. Throw in some Possessed or obviously Chaotified models, and I can use the CSM 'dex.
Mercah; I do believe Ahm gettin the vapours!
GFP