I'm pretty happy with Kaspersky -- that's what were running on most of the machines here at work.
Aside from the occasional hiccups with AVG (like that one where it flagged VFP programs or an occasional DLL within Windows as a virus), it's been working OK so far.
Haven't had much problem with Bitdefender, though on a couple machines here that have it are running a bit sluggishly (though I suspect much of it has to do with having only 512MB of RAM and some multimedia software installed on them -- they didn't run slowly until someone started installing the multimedia software).