I find nurgle interesting with his very friendly and fatherly persona. It's such a contrast to the other gods. Probably my favorite, though death guard being all gloomy and self hating was kinda meh. But then I just decide my guys are the one happy death guard warband and stop caring. I also like the aesthetic of the deathguard, even if I'm not as big on the straight gore aspects.
Khorne is interesting, I like his lore but again his cult troops leave something to be desired. I like the whole honorable combatant shtick though, and he has the best looking demons
imo.
Tzeentch suffers from both being an unrepentant

in the fluff and having a really boring cult troops. I'd enjoy him more if we saw more the mutation focus brought out,
40k could us more mutant representation on the tabletop.
Slaanesh could be cool if they ever locked down his/hers domain properly. I guess excess and perfection kinda fit together, perfection indicating an excessive focus on a skill or aspect, but it's still rather odd. Noise Marines are fun, but again they don't always fit well with the rest of their legion (both in rules and in general theme).