Most new series get one of the big hitters to appear in their first or early issues – it boosts sales and cements the new title’s position in the DCU. While I’ve no statistics, I’d be willing to bet Batman, probably followed by Superman, is the character that has the most guest appearances in new titles.
Working through my old comics, I think I’ve found the best Batman guest appearance ever:
Garth Ennis and John McCrea’s Hitman #1 where star Tommy Monaghan gets to throw up on Batman’s boots.
Genius!
Crisis on Earth-Prime 