Half-Life 2's Highway 17.
Even when I was playing HL2 for the first time, I had generally been pleased by the game, until that point. The first part was decent, especially when the magnet fails and you (and the buggy you're in) end up falling upside down and Antlions started attacking, that was cool. After that point, however, it was merely driving on a mostly deserted highway, with the exceptions of a few abandoned (or occupied by Combines) shacks or small houses that you could "explore" and find some ammo in... that's about it.
The actual atmosphere wasn't bad, you could "feel" (if still immersed at that point while understanding the story) that perhaps the rest of the world outside of Combine-controlled areas would have ended up like that, almost barren of life, silent lest for the sound of the wind. It was decently done, but it wasn't the superb experience I had in the earlier levels. The Combine Gunship battle scene was also decent, but alas it was the climax of the otherwise quite boring Highway 17, and then from there you move to the Sandtraps level, where things don't exactly get any better. Additionally, you guys surely remember how "physics-based" puzzles were the fuss of the game (and the day)? I admit some of those puzzles were kinda fun, although most of them were pointless and simply there to remind you that you had a Gravity Gun, or that the game used physics on lots of props. One puzzle in particular had me wondering if the developers had made it indeed just to "show the physics off" or if they just told themselves that a bunch of Combine grunts lost their brain for a moment.
It's the part where you drive up to a force field you can't drive through, so you follow the cables and they lead to a very stupidly placed Combine vehicle that happens to be just well stationed at a specific angle so that if you remove the blocks from the wheels that oh so conveniently prevents it from rolling down the ravine just behind it with your Gravity Gun, then the vehicle slowly but surely does indeed roll down and falls in the ravine, cutting the cable and the power to the impassable field in the process. When I saw that and finished the "puzzle" I literally stopped a moment and remained silent, looking at the screen and thinking... «The Combine were reduced to doing that? To power up a force field? And of course instead of stationing the vehicle to a safer location they decided to place it just two meters away from the edge, and place wheel blocks to prevent it from falling... those same guys wiped out the World's military within seven hours...» so, suffice it to say that it kinda breaks down any immersion you might have if you do pay attention to "details" like I do especially in linear story-driven games, be it FPS'es or not.
The game as a whole was good, it really was, although yes the ending was extremely anti-climatic, I would compare the "usefulness" of HL2's ending (referring to vanilla HL2 here, not the Episodes's) to that of Borderlands', it's that bad. But anyway, the "worst moment" or worst time spent in HL2 was all of the above, and a bit more as well that I won't bother describing. Let's just say that a 50% shorter Episode Two completely wiped out vanilla HL2 in terms of enjoyment and especially in terms of actual story progression.