This is a really arduous task. I've answered such things before, but always with great difficulty. I've played many great games, but even more shitty ones. I'll list some
of my all-time favorites, but not in any particular order.
1.) Halo: Combat Evolved
2.) Jedi Knight: Dark Forces II
3.) Diablo II
4.) X-Wing Alliance
5.) MechWarrior 4: Mercenaries
X-Wing Alliance, a game I miss dearly. It was one of the final giants before the death of the 'joysick age' and consoles rose to 'power'. The multiplayer community was absolutely steller, and had a wonderous blend of high-competitive zones as well as a massive population of players who basically roleplayed as starfighter pilots with writers creating all sorts of amazing missions/stories for their squadrons to play using the games custom battle feature.
You could basically set up fleet battles with npc ships along with people controlling fighters themselves with all sorts of objectives/environments to play. Or simply have 1-on-1 dogfights in an asteroid field or something. It was a somewhat limited tool, but it was just complex enough for all sorts of epic battles to take place and a mission could be built in a matter of a minute or two.
Even though I don't play XWA anymore, it will always be, perhaps, my all-time favorite game ever. They say good things don't last forever, and this is no exception.
I played this once for a couple hours over my friends place using only a shovel I picked up from the beginning. It was a wet noodle later in-game, but it was awesome.