Depends on what you want out of the game, how you like to play, which parts of the stock game you would most like to change or enhance, and what types of things you'd like to see added.
Some of what each supermod has to offer can probably be gleaned by reading the stickied threads for them in the Mods Workshop forum, or you can ask specific questions there of the players who use each one.
If you're lucky someone who's played all of them will stop by and give you a quick rundown of the differences between them. Sadly, that person is not me. Yet.
