*makes list* Well, of those choices, maybe Larry from SMB3. Have to sometimes run under him when he jumps, very satisfying to get the first wand back. I may have also been on a SMB3 high at the time, because I had the player's guide (I can't frickin' believe I got rid of that) and thought Larry looked cute with the wand in his hand. Always enjoy seeing those Koopalings, so it's kind of a nostalgia thing.
Runner-ups:
SMB: Bowser (this should be first as its the most challenging and imposing, but it didn't wow me in presentation)
SML2: Bird at Tree Zone (more dramatic than Larry from SMB3, but not as challenging)
SML3: Spiky Turtle thing (pretty cute for a boss. Helps that he came at the end of Rice Beach, one of the better world names in the series)
SML: Lion thing at end of World 1-3 (annoying as heck, but I enjoyed the pain sounds it made when you hit it with fireballs)
SMW2: Burt the Bashful (sure, the Super FX or whatever effects were impressive at first, though I don't care for how you defeat him. Found Salvo the Slime more satisfying because you're constantly chipping away at him)