I'd have to go with Doom 2. The balance between weapons and enemy types made it a beautiful ballet of bullets, plasma, fire, and lightning. I still haven't played a game that had such a perfect balance between the weapons and the enemy types, every weapon has it's uses and every enemy has it's specific strategy to take down. Add in multiple enemies, such as a room of Revenants, Demons, Cacodemons, Imps, and Chaingunners, and a few Barons of Hell, and you have multiple enemies that attack in completely different fashions, do different damage ranging from pitiful to almost instant death, hitscan enemies that can hit you instantly and enemies with homing rockets, or that fire projectiles you can strafe in to if not careful. The pain animation is interesting as well, because if it's triggered the enemy can't attack. Some enemies have a high chance of triggering the pain sprite, such as the cacodemon, imp, others it's very rare such as the Arch-Vile, Cyberdemon, Baron of Hell. You can learn to use that pain state to stop enemies from attacking, but not kill them while you concentrate on harder enemeis. It requires quite a bit more on the fly planning and using the optimal weapon in a situation that any other FPS I've ever played. Plus doing a Tyson run against three Baron's of Hell at the same time armed with a berserk pack is just a hoot. The art style helps quite a bit too, it's not a work of art, or a great story, but it embodies what an action game should be. Varied, frenetic, and packed with awesome-sauce.
