Master of Cunt-Puppets
I wouldn't personally recommend it without the mods, because of how night and day it is with. I couldn't imagine playing with the base classes/campaigns.
I've recently started fucking around with some of the music mods to add in soundtracks from other games. The base game's soundtrack is solid, but there's nothing like the classic Deus Ex score when a fight breaks out coming on after some MGS3 sneaking music while you're in concealment.
I played through the whole game, then replayed it with each mod. But I'm probably a masochist. I played Chimaera Squad twice.