Voted Tennis, though boxing is a close 2nd.
Golf & bowling are 3rd/4th... I think ultimately I would choose golf over bowling but there were other better golf games on the Wii, bowling I guess was surpassed by Wii sports resort version but the original Wii bowling did it's job almost perfectly anyway, perfect party game for including non-gamers.
Didn't really do much with Baseball, I imagine it was popular with Japanese gamers, maybe American gamers too but I would think they would want something with more depth. As it is I think I played more of the Wii Play billiards game than I did of Wii Sports baseball.