I'll be nominating X-Com: UFO Defense. One of the all-time greatest turn based strategy games that really defined the tactical/turn based/RPG genre that had some other great games in it (Jagged Alliance etc).
Destructible environments, squad development, research, ship building, tactical combat, tons of items to build/find/use, just tons of great mechanics. Every turn for each character was broken into Action Points, and each action you took (throwing a grenade, moving, ducking, taking a well aimed shot vs a quick shot) had a cost. Just so much to get into and pour over, but it always remained pretty accessible. I'll never forget the excitement of touching down to a crash site with a fresh squad of rookies in some North American farmland.