HoD 2 for me. I remember when I first got it with a gun. I got it home played it straight away and I grinned. Just a shame Sega didn't really do anything original with the lightgun genre or release any more compatible games. I always thought some sort of co-op game with one player controllong a vehicle or mech type machine with a second player using the gun.
I do enjoy Sega Rally 2 as well but it's not as much fun.
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