Ultimate X-Men is enjoyable, but doesn't get my vote due to occasionally seeming constrained by what it's 'supposed' to be. Like the creators want to tell a cool story, but have to do it within six issues and have to do it with some new version device from Marvel proper. The Phoenix saga is not a story that takes six issues to tell; do something new with Jean Grey. A psycho serial killer out to kill mutants is one of those simple but interesting ideas; making him Ultimate Mister Sinister just seemed unnecessary. It also occasionally seems to skip over bits just to move the franchise along (perhaps to bring it closer in line with the movie); Nightcrawler popped up as a guest in their Weapon X arc, then all of a sudden he was an X-Man. Rogue just 'turned good'. And why the fuck is Juggernaut in the Brotherhood of Mutants? He's not a mutant.
It should be added that I love the X-Men. They're probably my favourite superhero characters. I love the writing of Millar, Bendis and Vaughan. I love the concept of the Ultimate books, so I can read the adventures of these characters without having to know what happened in issue 73 of Uncanny X-Midgets in the eighties. So it pains me to admit to aspects of Ultimate X-Men that I've not enjoyed as much as I expect to.
Ultimate Spider-Man, I like. But the fact that I can't think of any comments to add beyond that probably says something.
The Ultimates gets my vote. It's the most original of the Ultimate titles; it's not trying hard to be The Avengers with some modern day celebrities thrown in to make it kewl. Millar has a genuinely original and interesting take on these characters, and he writes them damn well. Ultimate Cap is NOT the same character as Marvel Proper Cap, but I find him even more interesting. And Hitch... what does one have to say about Hitch (well... maybe "draw faster")?