7.62 High Caliber. Yes, that is literally the name of the game. Flaws? Let's just start strong with its name. It's downhill from there. It's an obvious prototype, it's unfinished as all hell, and it has a guaranteed memory leak that will crash the game.
And yet, there's really nothing else out there like it (save Marauder, aka Man of Prey, which is a much more stable, watered-down, linear experience, though still pretty cool in its own right). It's a pausable real-time squad-tactics game with a lot of control minutia, and an emphasis on simulation. As in, "Let's set the game speed to ultra slow-motion to watch this spray of bullets ricochet off the wall and maybe hit some dudes around the corner." Fun, fun stuff.
EDIT: Castlevania 64 was a terrific mood piece. I gave up on it as a kid because I couldn't make my way down the cliff on the opening stage. Because it was a rental, I never played it again. Years later, I got to enjoy the humiliation ritual of watching my wife beat the game rather handily. At least she managed to get the bad ending on her first run because the vampire hunter dude died.
Fantastic soundtrack.
Another great but flawed game on the N64: Hybrid Heaven. Nothing else in the world like it, and shockingly fun in multiplayer. Especially if you play against someone who loves to call out their moves as they spam them over and over again (my wife). "Boston crab! And another Boston Crab! Get up, get up. Boston Crab!"