Having just made a thread about not-so-great games, I thought I'd follow it up by asking the RPS community to list their favorite game(s) and, if you want, give a brief explanation as to why you love the game. I searched the RPS forums extensively to make sure this topic hadn't been posted before, and, surprisingly, I didn't find a thread like the one you're currently reading. If I missed it, please let me know and link the thread. Anyway, here's a list of some of my favorites:
Strategy (just lumping together RTS, RTT, etc. for simplicity's sake): Rome: Total War. The level of detail and strategic depth of this game was simply astonishing for its time. I still really enjoy R:TW to this day.
FPS (Single-player): Doom (I+II) and Blood. There are plenty of FPS games, like Half-Life, that I don't think I can even compare to the "old school" shooters I've listed, in that games like Half-Life and games like Doom attempt to accomplish totally different things. More to the point, Doom and Blood achieve the best and purest brand of first-person shooting that I've ever encountered. The levels, weapons, enemies, and basic feel of the combat in both games is absolutely superb. Most importantly, the act of shooting feels immensely powerful and satisfying in the games.
TPS: Max Payne. Some will argue that Max Payne is essentially an FPS, but with the camera backed up a bit. While this is essentially true, I prefer the controls and mechanics of Max Payne to any of the "true" TPS's I've played, most of which feel unnecessarily slow and cumbersome. Max Payne combined a phenomenally dark atmosphere, satisfying, tight shooting mechanics, and exceptionally challenging levels to produce a rewarding and enjoyable experience. Despite its extreme linearity, I've played Max Payne through about three times and had a lot of fun during each playthrough. The game's lasting appeal is, to me, a testament to its immense quality.
RPG: Fallout. The thing that initially set Fallout apart from the pack, and still does, is the weight the game gave to choice. Unlike most RPGs (Kingdoms of Amalur, TES, etc.) which allow the player to turn characters into supermen that can do anything and everything, Fallout forces the player to make difficult and meaningful character development decisions that create a true, specialized role. Resources are also very limited throughout the game, which adds an additional element of complexity through involved inventory management.
Survival-Horror: Amnesia: The Dark Descent and Silent Hill 2. Survival-horror doesn't exist in the mainstream anymore. Resident Evil 4 supposedly "changed" survival-horror, but in truth, RE4 is nothing more than an third-person horror shooter. Since RE4 is part of a series that was previously composed of true survival-horror games, most people, incorrectly, considered RE4 as such. There is no scarcity of resources in RE4 and the enemies are no more a threat than they are in a typical action game. The culmination of these elements not only removes any survival element from RE4, but also castrates the horror. Conversely, Silent Hill 2 provides the player with very few resources, limits, some would argue cheaply, combat effectiveness, and creates a dark, penetrating psychological horror that relies heavily on a perpetually unnerving atmosphere, rather than cheap jump scares. Amnesia takes the Silent Hill 2 approach to survival-horror even further, by completely removing all means of fighting/killing enemies. All enemies encountered in Amnesia are, as a result, extremely scary, in that even a single enemy can easily kill the player.
Multi-genre game: System Shock 2. A lot of people would probably choose Deus Ex for this slot, but, in my opinion, System Shock 2 is undeniably better. Combining first-person shooter, role-playing game, and survival-horror, System Shock 2 expertly implements the various constituent elements of its design. Scarce resources, dangerous enemies, and complex inventory management establish great survival-horror gameplay. The role-playing is similarly fantastic, as, like Fallout, the skills system forces the player to take on a specific role (hacker, shooter, psi user, and others) which leads to difficult decisions as to the best course of development for your character. The shooting isn't very good, but it's far more functional than Deus Ex. The Dark Engine does make the melee wrench a rather satisfying weapon, however.