I think I find it hard to define exploitation films. Sure, stuff like Faster, Pussycat, Kill, Kill and Shaft are easily definable, but I have to say I've never gotten into such movies. I've seen both, but they haven't left a big impression. When looking for lists on line, I see movies like Freaks and Halloween, which I consider horror. I see films like Repulsion or Reefer Madness and I think of them as arthouse or cult. Unless they're really known for being exploitation, like Russ Meyer or Blaxploitation, I don't think of them as such.

So, based on a somewhat loose interpretation of exploitation film, my favorites would be Freaks(1932), Slap Shot(1977) and Halloween(1978). I love 80's slashers, but don't have the same feeling for films like Last House On The Left(1972) or Cannibal Ferox(1981).
I think I'm Gonna Git You Sucka(1988) is hilarious and Deep Throat(1972) had some legitimate laughs.
I saw Reefer Madness (1938) with a few guys and a few bones and thought it was hilarious. A few weeks later, we watched it again with a few beers, no pot, and it was boring.

Wikipedia lists The Wild One(1953) as a biker exploitation film. That's a great movie!

I like giallos, most notably Deep Red(1975), Suspiria(1977), Tenebrae(1982) by Dario Argento. I remember I thought one of them was very good, but can't remember which. Maybe Tenebrae. I've seen Mario Bava's Twitch of The Death Nerve(1971), but I mostly liked it because of the well known scenes that were copied in the Friday The 13th movies.