Looking for a FPS with a good single player campaign. Suggestions?

[Reasonable]

Half Life 2 (and the Episodes) are the king of FPS SP campaigns.  You need to get yourself the Orange Box.  Also, set up a Steam account and get Half Life Source - it'll run on pretty much any PC and while the graphics are dated it remains a fantastic SP campaign.

Really, very few - if any - SP campaigns seem able to match the Half Life series for innovation.  Most modern FPS just take elements from it and apply the 'make it bigger' principle but they don't actually add anything new really.

The Orange Box plays pretty well on 360, too, and you get Portal as well, the best - if short - FPS campaign (with no shooting though) of recent years.