QuickTime is the best, because it brings profits to Apple, is infuriatingly "user friendly" and it has nice exploitable security holes!
Seriously. What is "best"? Do you want the player to have skins so you can make it a huge pink semi-transparent tentacled Hentai character? Does it have to play videos? Do you want to play audio DVDs? Do you want to play DRM content from online shops? Do you want to play open formats like OGG or FLAC? Are you interested in plugins like sound "enhancement"? Ripping CDs? Recording music? Do you want pretty visualisations? Do you want something that consumes 100MB or RAM or would it be OK to have something that fits in less than 1MB? Do you want to run it in WIndows 3.1, Windows NT, Wndows 95 or maybe WIndows XP or Vista?