The original CoD is still my favorite MP shooter. I never played United Offensive though, I own it, just never got around to installing it for some reason. CoD2 was pretty good, I didn't like it as much as the first but I moved to it anyways because of the improved graphics and larger player base. My biggest issue with CoD2 was the disappearing xhair when moving, also hated the addition of the trench gun. I didn't like the grenade indicator at first either, but it actually grew on me because of the excessive nade spam CoD is infamous for. Oh yeah, and no more health kits...
The only MP shooters I've had close to as much fun with since CoD are TF2 and Uncharted 2. If Uncharted 2 had LAN support I think it'd be a competitive league game like Halo and Modern Warfare, it's ridiculous fun and would rival any PC game if it had KBM support. TF2 I put a lot of hours into, but haven't touched it in a really long time. It's a little too casual to play seriously. I never feel any sense of accomplishment when my team wins a round in TF2, it's just something different to mess around with once in a while.
I've played Quake 3 and 4, UT2K3 UT2K4 UT3, Battlefield 2, CS 1.6 and CSS, Painkiller, all the Halos, Modern Warfare, and Combat Arms, but I couldn't get into any of em as much as CoD. Combat Arms deserves a little credit for being free, I'd actually rather play that than Modern Warfare, so it's worth a try. I think the MP portion of FEAR actually went free to play a long time ago, too. That was kind of fun for a while, but I gave it up pretty quick, sliding melee attacks were an instant kill and abusable around corners or in hallways.