My Raspberry Award goes to Portugal; rather contradictory traits, not so fantastic UU, at least not for my style.
Other candidates:
Hittites: In real life a civilization I am fascinated by (I am a historian), in Civ3 usually an uphill struggle. Akward traits, mediocre UU.
Spain: Yes, I have already been rebuked by DocTsiolkovski in a former thread for this, apparently it is one of the best for the 20k culture victory. However for somebody on my level (I have played for about half a year and am now usually doing my damage at monarch) with the accordingly lack of sophistication that sort of considerations are secondary. I don't like the Conquistador, I think Spain should have the Tercio as UU.
Rome: Why the heck aren't they militaristic and industrious? Then I would gladly play them. As it is now, I have yet to get anywhere with them.
Korea: I wish they were good, but you can wish in one hand and spit in the other....The few times I tried, I lagged too far behind to have a decent chance to catch up, and their UU confuses me.
America: I will probably like them when my playing skills are improving, right now I fail to see the charm about Honest Abe. And that UU is a waste.
Worst opponents:
Iroquis: A real scientifical and economical powerhouse and with a great UU. I have lost quite a lot of games to them.
Sumeria: Also quite good, great traits and due to their UU the AI are capable of expanding very quickly with this civ.
Zulus: Picks up lots of techs early and usually disposes over a tremendously big army.Which they like to bully me with.
England: In my games, for some reason, they are always very strong.I like to play them myself on archipelagos, but why they are so powerful even on pangea is something that puzzles me a bit...