For multiplayer, Quakeworld is pretty untouchable. The physics are way more outlandish than modern FPS, you can bunnyhop around at insane speeds, bounce people in the air and glue them to the ceiling with shaft (thanks to instant weapon switch). There's none of this waiting around for the round to end lark, just full on action. A typical number of frags in a 20min 4v4 team game would be around 300-400.
A word of warning though, the learning curve is almost vertical and even after playing it for a year or two you will likely still be regarded as a newbie, getting completely annihilated most of the time. The game gives players so much freedom that skill differences are exaggerated, and it's so brutal that most new players find trying to compete futile and give it up quickly.
On the singleplayer front, Serious Sam Gold, Serious Sam 2, and Painkiller are worth looking at.