When polymorphing into anything non-humanoid, be sure to remove all clothing and armor before doing so. You'll destroy it all if you don't with the exception of amulets and, I think, rings.
Black Dragons aren't an optimal choice, since their natural AC, which can't be improved with armor, is too high to be worthwhile. Also, the dragonbreath consumes an awful lot of MP. However, the disintegration breath allows for making short work of most Quest bosses.
Vampire Lord and Master Mind Flayer are the choices considered most optimal for controlled polymorphing. I prefer VL's, due to being statistically superior in nearly every way, having max strength, health regeneration, natural flying, undeath (and thus, natural immunity to instant death spells and effects), magical breathing, a level draining bite attached to attacking normally, and some other things I'm sure I'm forgetting.
EDIT: Also, I might be mistaken, but you should not pray to a lawful or neutral God while poly'd into a VL. They will be angry. I'm not sure if the same counts for chaotic gods though.
MMF's are also fun, because even though they don't have as many advantages, they have 4-5 brain eating tendril attacks attached to every normal attack. This will make short work of any enemy with a brain. Does not work on brainless opponents though, ESPECIALLY acid slimes and the like, as they splash you can can deal impressive amounts of damage in a short period of time from just splashing.
Also, both biting and brain sucking count as touching a cockatrice unprotected, which will stone you instantly.
There's also a variety of angel that you can poly into, that doesn't have as many advantages as either, but can dual-wield, and has a kick attached to their regular attack, which doesn't have the drawbacks of biting or sucking so long as you're wearing shoes.
I loved polymorphing SO MUCH.