[promo]Duck Hunt! 1P vs 2P + duck-control makes it a 3 players game. Very fun to defy your friends and decide who is the best hunter. Problem: you need 2 light guns in order to play it like this. Check the BCN User Meetings photoshoots. [/promo]
I love also Fernando Martin Basket Master (the second version), and -of course- Track and Field (Hyper Olympic) 1 and 2, and Yie Ar Kung-Fu 2 (specially when you combine it with Game Master so that you can select ANY of the different enemies as 2P).
But again, the MSX multiplayer game is IMHO Gauntlet. And Psycho Pig UXB and ... There are too many of them!
Can't remember the name. It was a (MSX1) game with tanks in a labyrint. 2 players at a time in (a vertical) splitscreen mode. Playing against a cpu player was possible.