Herzog Zwei is a weird one. I don't even recall how I ended up with it on the Megadrive, but it certainly doesn't seem like the sort of game I'd have sought out at the time. It must have been the first RTS game I played (indeed it's often touted as being one of the unsung pioneers of the genre). I didn't find it much of a one-player experience, but over one summer a friend and I discovered it together and played it to death, even taking days off school to engage in marathon sessions.
It's a fairly straightforward game of conquest, each player starting off in a particular main base on a map, with a number of unclaimed smaller bases between them. These bases have to be captured by producing enough troops to fill them and various weapons and support vehicles to defend them. The player has an active role on the battlefield controlling a jet plane / giant robot transformer which can carry these units into position or directly attack the enemy. The player has limited flight fuel which can be replenished by flying over a friendly base. Victory is won by capturing the enemy's main base.
The interface is simple and the action fast and furious. Because the game is played out on a comparatively small scale the back-and-forth is very intense, with key bases changing hands constantly until a crack in the tactics is exploited and an upper hand gained.
Along with Streetfighter 2, Herzog Zwei is probably my most-played competitive two-player game. It really does need a human opponent to bring out the best in it, but it deserves its unshakable place in my 100 just for that summer and for the promise of a worthy opponent in the future.