True test IMO is how often I come back to the stock single player. Never replay through single player games, however I've played through countless missions in Far Cry time after time - the first few especially.
HL2 I played through the coast level which I loved (again, give me a huge vast landscape and I'm hooked; I don't get bored of that easily) once or twice, exploring around, and that was it.
Doom3 was helluva fun, however I don't think I can replay it through without some serious mods.
This is not to say Doom3 is clearly the worst of the 3, just got my money out of FC the most.
Besides: 3 words: Far Cry Coop. A truly amazing experience when you've got your teammate in the same room, and cruising along in a gunboat or jeep. (Going off topic, but the apex of gaming will probably occur when you can do something like 32 player coop in a massive war)