Hmm, tough one.
Why not rent out some of the online enabled titles from Blockbuster to see if you like any of them.
Also GAME highstreet stores have I believe a no quibble returns policy so you can return any title that you think is a real turkey.
That said, let me run through the titles I have:
Twisted Metal Black Online (fired it up maybe once or twice, haven't got into it yet).
Fire Warrior, reasonable FPS but have yet to play it online.
Socom I (have, but haven't played yet).Consensus appears to be buy Socom II instead..
Destruction Derby Arenas, loaded this up once, haven't looked at it again since. I just don't have the time!
Need For Speed Underground : Hmm, not bad. Graphically it's no GT4 (or even GT3 come to that) but the online play seems pretty solid.
Midnight Club II : Not bad, but not fantastic. Also a bit of a disappointment in the graphics department.
Hardware: Online arena : This is actually my favourite. Instantly pick up and playable, it does have an offline mode but this is really only for training purposes.Great fun to taunt your enemies via the USB headset either when playing team games or just in a free for all.
Tom Clancy's : Ghost Recon: Only just bought this so it's a bit early to comment. Working through the training missions at the moment, seems like good fun and I like ordering the squads around via the headset. Even if they do seem to have cloth ears sometimes.I mean honestly, who doesn't know the difference between "hold" and "advance"??
As a member of the PS2 online Beta test community, I've been lucky enough to get some of the above titles free. I think it's fair to say that newer titles are getting better in their use of online play.
If the online bit is done right you shouldn't have to mess around opening ports on your router/firewall to make voice comms work. These should be load and play (this is a console after all, not a PC), some titles manage this better than others.