Master Transmogrophist: a Wizard PrC that gets 6/10ths casting that gives awesome buffs to Polymorph.
Sadly, Shapechange is better than the PrC, but still...
The ability to Polymorph with bennies (like adding Su abilities from other creatures) is win, especially when it is based off of your stats; instead of your ability scores adjusting for being tiny, you instead get a -8 to Str.
Throw on a scroll of Giant Size... and you are an insanely strong Tiny creature with, I don't know, Choker's Quickness.