Using sex position names from http://www.sexinfo101.com/allsexpositions.shtml or any other sex position guides you know about, what are some favorites?
Personally, I dig the "Folded Deck Chair" position:
http://www.sexinfo101.com/foldeddeckchair.shtml
This is a bit of a tough one though because it requires a lady who is both very flexible and can take very deep penetration, which is what really gets me going. Only a few providers I've seen can fully do it without too much discomfort and I certainly don't want to hurt anyone but for those that can do it, it is my go-to.
For a little less extreme variation, a version where she rests her feet on my chest instead of sticking her legs up in the air is also very good (not sure what this one is called and sexinfo101 doesn't seem to have an entry for it).
For those that can't take either of the preceding ones, regular "Deck Chair" usually works well:
http://www.sexinfo101.com/sp_deckchair.shtml
Despite occasional forays into cowgirl and various forms of doggie, I'll almost always go back to one of these to finish.
What do others like and why? Providers or clients.