Oh Mr Ben you are in a contentious mood! What about Half-Life 2 as a solo game? The design is sublime, the puzzles are smart, it has variety, structure and pace.
Agreed though COD 4 is really just a big old rollercoaster. It's very fixed, very limited and full of IW usual tricks and shortcuts.
HL2 was alright from what I played (which was a fair bit on my housemates computer), but it lost some of the awesome clean fun of the Vent Shaft Crawling Simulator '98 which was the original.
It got a bit Oooh look at our physics engine for me.
The best 3 First Person games are still Deus Ex, System Shock 2 and probably the original Half-Life. Although of course Half-Life sucks fucking goats once you get to the alien world. Fucking yawn.
