Although I've never really played Q2 except for couple of levels (exception: SPAQ: LE, which is basically Q2 with ActionQ2 weapons and physics), I consider the gunners most effective. Parasites freak me out, and I don't really like the medics, either.
Apart from that, killing a tank with a single knife (slash) and by kicking him gives me some great "kick"... Too bad they're so slow.
Please bear in mind that I'm not used to fighting these creatures with the normal Q2 weapons, so perhaps my views don't really matter here.
Once I get a new video card, I'm going to give COOP or Die a try, I really like the concept. Yet, I'll have to admit that I'm more for the physics and armoury of AQ2, so COOP or Die combined with SPAQ: LE would be a God's gift for me. Well, or maybe not... It would be pretty much impossible to beat the game with a single life that way.
Barracudas used to scare the living hell out of me until recently, when playing SPAQ on some level... I think it was city1 or the level before it. I fell into a "river" full of barracudas, and armed with a single pistol and a knife, I fought my way out of there. I felt so good afterwards that those things don't boggle my mind like they used to do.
Sorry for going off topic here!
Oh, and I voted for the gunners.