Geez. I dunno.
I think, for USM, I'm going with Learning Curve. That's Bendis at his best. Histerical, tense, and well-plotted. No cheating, no nonsense, all brilliant. It was this trade that actually got me into the Ultimate universe. So, I love it.
For UXM, it's tough; but I think, over The Most Dangerous Game, I gotta go for The Tomorrow People. Very well done.
For UFF, I can't choose between Doom and the N-Zone. It's too early to tell for me.
The Ultimates... depending on how Gods and Monsters ends, it's either that or Super Human, the first six-issue arc.
As for the mini-series... Ultimate Daredevil & Elektra, Ultimate Elektra, Ultimate Team-Up, Ultimate Nightmare, Ultimate Secret, Ultimate War, and Ultimate Six - I want to go with the Shang Chi issue of Ultimate Team-up, which I loved - but I'm going with Ultimate Secret. One issue, and I'm already in love with Captain Mahr Vehl.
Overall? The Ultimates' "Super Human" arc does it for me.