You can't really say who's the best fps developer, i mean valve made some crazy games.
Crytek made the amazing engine for Crysis and other games.
Infinity Ward is slowly starting to get up there with the modern warfare series, so i can't really say who's the best
Most of them are great.
But for PC i play Counter-strike 1.6 / Team Fortress 2 and Valve made those, so i guess right now Valve's my favourite PC FPS developer.