No such thing as best, it's all scenario
I might use Tier Tacticool for a death race
But I'd use H&K for a city RPG
I might use gravity cats weapons with his vehicles for fortwars
Or I might use Tier Tactical for it's modular ability (Frog's weapons that require T+T) for a sci-fi shooter
I might use BF3 if I want a simplified combat system for a small DM
Or I might use block combat for something a little more advanced.
Appearance wise, I'm a fanboy of H&K
Tier Tactical takes the cake for modular ability
BF3 is pretty good for simple stuff
Haven't tried quake yet. Looks cool but the weapons sound rather stupid, which would be good for a comical DM.