I fix computers as a side job and most of the time clients request some sort of an antivirus installed. So I've been installing them per their request. The thing is, most of them don't know what's good and what's not, so over the years I've tried them all (just because I could get them free after mail in rebates).
As of 2010/2011 I would rate them as such (based on system usage and detection/features)
1. Kaspersky (although the 2011 for some reason starts to bug me with random system scans once in a while)
2. Nod32 (used to be my best, but has fallen a little behind Kaspersky)
3. AVG (honestly, i'm not sure why people complain about this one, seems to work fine for me, I'm talking about the paid version)
4. Webroot (interface is kinda cheesy, but program itself does not use a lot of resources)
5. BitDefender (small and not complicated)
6. TrendMicro (I was actually surprised by this one. I thought it was a lot worse)
7. Avira
8. Avast (I know some might disagree but I've had several instances where it wouldn't update and so forth, it started to become annoying)
9. Norton (2009 and up only. They really reworked the whole application. At this point it's comparable to the ones above)
10. Panda (ok, but a little too much overhead for my taste)
And now, something to stay away from (bad experience)
CA Security Suite (had many instances where it would block several ports, even access to computers on the same workgroup for no reason)
McAfee (someone already mentioned)
ZoneAlarm (don't even get me started on that one)
There are many others and they might perform equally well, or equally terrible, but those are the only ones I've used over the last couple of years. All installed on clean OS's.
Also, as other's have said, stay away from Security suites. Some of them try to block/filter things and it just leaves you pissed off. Some don't even give you the option to "unblock." If possible, just buy the Antivirus/anti-spyware, but not the whole security suite.