Fun fact: The Ultimates is the only U.U comic that has been translated into Japanese and released there domestically, with a wraparound ad using it to cross-promote The Avengers movie. I ran across it in a bookstore once, and will probably pick it up once I finish buying the manga series I'm reading. I kinda want to see how its cultural references are translated.
Anyways. Ultimate Spidey is actually what got me into comics in the first place - my high school library had a copy on the shelf, and I knew the title from the game. I was completely hooked, and I'm with Captain Canuck: I don't think it ever got bad, with the exception of the WAR OF THE SYMBIOTES arc. That story is atrocious, and for my money, the only downright awful story told in the book's long run. It definitely had a bit of slump there for a while, but I felt most of the arcs from Silver Sable through Ultimate Knights were just mediocre, not bad (yes, even DEADPOOL, and I'm actually in the minority that didn't outright despise the CLONE SAGA). My favourite arc is easily VENOM, but LEGACY is a close second. And ULTIMATE SIX is what piqued my interest in The Ultimates.
And I'm glad it did! The Ultimates is excellent, and I can't say much that hasn't been said about it already. Whedon's intro to the hardcover sums it up nicely: Millar deconstructs the idea of a superhero just enough for you to worry about the team, but not so much that you're not cheering for them when they succeed against the odds. Ultimate Cap is what made me interested in the concept of Captain America, and though I've since come over to the boy scout version some (largely thanks to Brubaker and the Cap movie), I have a huge soft spot for Millar's nuanced man-out-of-time, to the point where I get embarrassingly defensive when people refer to him as a douchebag/***hole/whatever. And then the sequel! Ultimates 2 is such a tense, paranoid piece of writing, and is such a great follow-up to the original both story-wise and thematically (but did anybody ever read that aborted series War Heroes that Millar did later, that he was billing as the spiritual successor? Boy am I glad that didn't end up being the actual Ultimates 3!). I felt a bit let down by the conclusion to GRAND THEFT AMERICA, but largely left with pretty great impressions. Honestly, these are pretty easily the best things Millar's ever written - to this day, I'm astonished that the same guy who wrote Nemesis managed such great character development and worldbuilding.
I recall being a bit put off by Millar's UXM, though, even when I first read it. If Ultimates is everything good about Millar's writing, then UXM has to be everything awful - 90's XTREEEME all over the place, absolutely no sense of character voice, all of it. I really liked Bendis' stuff though, especially that one-shot where Wolverine has to assassinate a young mutant and the crossover with USM, and of course Vaughan's stuff. But the thing is, the book was never able to find a consistent vision or voice, and so it just sort of meandered, with little to no larger purpose to any of its stories.
I've never read UFF. I did love the Galactus trilogy and Ultimate Human, though, and while I agree with DirishB that Ultimate Nightmare is the best, and that the three generally decrease in quality as they go on, I enjoy the final product as a whole just fine. Boy, do things go downhill from here, though.
I never actually read Ultimate Power until a few years ago, but man, was it ever awful. Artwork, story, all of it. I went into Ultimates 3 with a sort of cautious optimism, but I doubt I would have if I'd read that dreck first. And U3 was BAD. I can't believe so many issues were used to do so little. The plot was a confusing (and utterly pointless) mess, the characters were one-note at best, and any remaining realism in the Ultimate line was dashed on the rocks.
AND THEN ULTIMATUM WAS SOMEHOW WORSE. I didn't think I would ever encounter a comic as bad as U3, but there it was. I actually destroyed the second issue; I was drinking with some friends, got on a rant about the series, and then just fell silent for a minute, looked and the book on my table, and tore it in two. Garbage. (I...I promise that makes me sound more drunk and more violent than I actually am).
I'll take a breather there, before getting into the later stuff, because this post is already enormous. I think, though, that this shows just how much the U.U means to me. It was my gateway into comics, and love it or hate it, it'll be a shame to see it go.