I've never had a true crush on a video game character, but there are some girls I think are cute or some such (well, obviously).
Vivian from Paper Mario: The Thousand-Year Door is cute. I've always liked red cheek highlights like hers.
Mimi from Super Paper Mario also...in her normal form, that is, not the other one. :D Same deal with the highlights and all.
I guess one could consider Rosalina pretty. My cousin said she was hot.
From non-Mario games, Amy Rose from the Sonic series looks kind of cute, especially in Sonic Unleashed. LEAF a page or two back is cute too.