That's a hard question. There are so many Wild Cards characters that I love... though, hell, I'm the editor, if I didn't love them, they would never have been included in the series in the first place.
Croyd Crenson would definitely have to be one of them. The Sleeper. All time best Wild Card character, the archetype for the series.
Fortunato, maybe. The heroic pimp was a real breakthrough character. Just think, Lew Shiner was twenty years ahead of those guys who won an Oscar for "It's Hard Out Here For A Pimp."
I also loved Jetboy. Although he only appeared in that one story, he set the tone for much that followed. Even so, I might have to pass him over for Gregg Hartmann. A good story needs a good villain, and Gregg was our best, with a story arc that went all the way from book one through book fifteen.