Lawlor. No question.
With Ritchie, I feel like I respect his abilities as a designer and his body of work a lot more than I actually ever enjoy playing one of his games. High Speed was probably the first game that really made me ignore the arcade machines I grew up with, but Lawlor's games were always more fun to play, and that continues to this day. Far superior theme integration as well (which has been noted).
The others have had their moments here and there, but have never been as consistent as the big two. Elwin shows promise, but I think its way too early in his career (and too recent) to claim favorites with him.