Yeah, I am gonna say Quake as well.
Quake, as a single-player game anyways, is piss-poor. To me, Quake is a techdemo that just happens to have an awesome multiplayer mode included. By comparison, Duke3D was the better game, even though it used an inferior engine. So I kept an eye on the Quake engine, to see what other developers would do with it. Some brilliant multiplayer mods were released, but as far as single-player content goes, only the unofficial (and taboo) Alien Quake mod was any good. The Nostromo level in that mod was awesome, it was the defining Alien-related gaming experience until AvP was released in 1999. Quake 2 was a more refined version of the Quake engine, and a half-decent singleplayer game was included, but it was what others did with the Quake 2 engine that caught people's eyes. Half-Life and Anachronox, just to name a couple of examples.
In Quake, you can see clear signs of decline, but for all the right reasons. Doom, despite all of its achievements and fun gameplay, was a 2D game seen from a first-person perspective. Quake was full 3D, so it was natural that some limitations were in place while the software was being developed and improved. But sadly, the elements that made the 2D (and pseudo-3D) games so much fun never did reappear in a true 3D engine. Doom could have dozens of enemies on-screen at once, just to name an example. Quake would break down and cry if you tried to have just one dozen enemies on-screen at once. It took Serious Sam in 2001 to bring back the mobs of enemies, but by then the "less is more" approach was a de facto standard.
Another example is the development process of Quake. Quake started out with a Norse warrior as the protagonist, wielding a big hammer during the Dark Ages and doing stuff that Thor is doing in the Hollywood movies. No guns. No futuristic crap. But somewhere along the way the focus changed, the plans were dropped. In favour of making Quake a Doom clone. I wonder why they did that...
And when "graphic accelerators" (also known as 3D graphic cards) were taking their first steps, the gaming industry was strangly harmonic to the development (and marketing) of those cards: Graphics became the focal point of games. This was apparent as early as 1997, possibly earlier. Up to that point, and that explains FMV games to a degree, the objective was simply to try to make games be on a level with the images and sounds you saw on TV, "multimedia-capable" was the buzzword IIRC. Sadly, far too many developers thought that the content should also match that of the television, which explains why it takes effort to name even 3 FMV games worth a damn.