Well my "favorite" is Cigar... I just loved to watch him lower that head and go for it. Heck, I get goosebumps even now thinking about him running.
He did allot for racing and helping to bring in new fans and that is always a Big + for racing as did most of the other horses that are being mentioned... This will always be a matter of opinion and the "best" will never get a 100% of the votes.