Laser Squad, playing couch hot-seat is what sent me down this path.

I really liked Jagged Alliance 2, Afterlight and especially X-COM: Apocalypse. Apocalypse had such radical departures from the first two Ufo titles, which did not make it very well liked among enthusiasts, in particular the real-time battle mode. But the game had such fun mechanics and steep difficulty curve, I really enjoyed the challenge of it, as opposed to getting another Enemy Unknown clone that was TFTD.