Birdo is more of a filler on my list because I don't like her in recent games. She clearly made a lasting impression in SMB2, and it's hard for me to hate the character overall because I like that game. I probably included her more because she made almost every best/worst list and generated a lot of debate. I would gladly put every baby character above her, as well as the fat, unoriginal Morton Koopa.
I think my biggest issue with a lot of Mario games is having to play as Mario when I want to be Luigi, Toad, Peach, or just about anyone else. SMB2 gave us that option with unique skills for each character (like ebin said). So, why is it the next game I can remember where you can play a one-player game as someone other than Mario isn't until Mario 3D World in 2013 or so? Perhaps more than anything, it's my hatred for Mario himself. Perhaps my next list should be "Top 1 Super Mario Brother" so we can debate between Mario & Luigi (yes, I'm including Jumpman as Mario for this purpose).
Either way, ebinsugewa brings up another one a lot of people seemed to debate: Waluigi. I love Waluigi in all of his purple-ness and evil Luigi style. To me, he is better than Wario in a lot of respects. Anybody out there have a strong opinion about either Wario or Waluigi?