Not an easy one to vote for, I liked a couple of games from them for sure.
Blood, NOLF 1 & 2, FEAR 1, and AvP2 (2001).
I'd say No One Lives Forever 1 & 2, along with Blood, and the original F.E.A.R., are probably my favorites. And I'd give AvP2 an honorable mention. My favorite is probably Blood, although that's also based on nostalgia. If I'm honest with myself I'd have to say that the first FEAR is maybe their best game overall, if I also include the tech behind it. I do say maybe... it's a tough one. It's either FEAR 1, or NOLF 2, tough choice. I really liked the first NOLF, but I think I preferred the second (admittedly I haven't played either of them in many, many years; I'd have to re-play both to recall which one I preferred overall). But I dunno... yeah tough picks, if I had to go with just one.
I could think of it with the context of the time in perspective when I first played the original FEAR. The thing is I've never really been a fan of games trying to be spooky. It rarely works (for me, anyway). I'd say back then I bought it simply because it was a 'new FPS game' to try. I'm pretty sure I was aware of the fact that it was sort of a "horror" type game (or that the setting was), but I just wanted to try out something new. Plus, if I recall correctly, when the first FEAR came out it was featuring some very amazing visuals for the time. I ended up completing it I believe 2, possibly 3 times. I recall wanting to play the game more than once from start to finish not exactly because I enjoyed the "fear" aspect of it (which wasn't too bad but nothing too crazy, a lot of it was predictable; I think I had maybe 2 slight jump scares... what worked perhaps was the 'apprehension' aspect, the "dread" in some areas, if anything) but because the A.I. in that game was incredible - at least as far as A.I. in FPS games go (and to this day it's still impressive).
But, if I analyze in a "whole package" way, overall enjoyment of any one of their games I played then yeah I think I'd have to go with NOLF 2. And Blood is a great Build engine FPS really.
The latest game I do own that was made by them was the first Shadow of Mordor game, with that famous Nemesis enemy system thing. I did play that one for a good 20'something hours I think. But at some point I kind of abruptly lost interest, stopped, uninstalled and never touched it again. I didn't "hate" it, actually, I did enjoy it to some degree. But I think overall I wasn't too impressed with the Nemesis system, realizing that in the end it was just a means to raid enemy camps, kill the "main boss" there, find another camp, do the same, find another camp and do the same thing again... I mean I saw little gameplay variety I think, from what I recall it felt repetitive too quickly. Although it's been many years of course (literally the same year it came out) and I never played it again since. They did make a sequel too I believe, but I completely skipped that one.