Variety (both in gameplay and design) goes to Sony, no contest imo. This includes not constantly whoring out your 2 decade old, boring IPs.
In quantity, Sony and Nintendo are comparable I guess, MS being behind quite a bit.
All three seem to have the "quality" part down. Sony might have a few more "bad" eggs due to more experimental&niche games.
edit: actually I have to say, personally Nintendo gets also less credit due to the Wii and now WiiU being basically one generation behind in terms of tech. In graphics, physics etc., their games will always be outclassed by far. All their Mario-esque games also have a design I don't find charming at all, so can't make it up with that.
Overall I'm much more interested in Sony's focus on experimental stuff, artsy stuff and cinematic games. It's my personal bias, but most of MS's games look like they are made for a US teenage boy focus group. I also don't dig most Nintendo's IPs anymore, even though (or because of) I've grown up with them, just can't stand the "characters" being not more than some themepark mascots.