My favorite characters are Ashley (for obvious reasons) and Lulu, she's too adorable in her little cutscenes ("If I'm gonna face Wario... I gotta carb up!"). In the past though I had a thing for Orbulon and Dribble & Spitz. It's a shame there wasn't a new vocal song in their new stage.
As a matter of fact, my favourite WarioWare character (besides Wario himself of course) happens to be Dr. Crygor. In the past, I used to be more fond of Jimmy T. and Mona, and although I still really like them, I'm more on the Dr. Crygor side by now.
Ashley is still my fav even if some unfortunate interractions with others (not Kyon in case he feels the comment is aimed at him) and the Smash tantrums did a number on my fondness for the character, lol.