The Adam & Eve line tends to be explicit without being completely nasty. When video porn attempts to be intentionally funny, it tends to fail horribly, and be neither sexy nor funny.
I could have sworn that Hoot Island had a list of good funny porn (this from a site which specializes in smiling naked women), but I can't find it right now.
While they don't have plots, the Shane's World series is quite funny and fun. It's bascially a bunch of friends hanging out and going on vacation somewhere, and they frequently have sex while someone tapes it. The non-sex portions are actually very entertaining.
Finally, if you have found a title you are thinking about renting, first go here:
http://amri.iafd.com/reviews.asp
and check out the review. It's a good way to avoid those unexpected DVDAs.