What is the best game/demo or 2-player-game to show to a non-MSX friend?

If you're talking about 2 players at the same time I can only think of Salamander, of which I've no doubt you've thought about that one already. I don't think there're many such 2-player games on MSX, perhaps because of the sprite limit. Another player could place himself next to you, which will cost another 1 or 2 sprites on a row, which may turn out to be too much.

F-1 Spirit, but you probably thought that one already =).

Try playing Penguin adventure with a 1-button joystick. The "other player" may have his/her finger on the "M" -key

Quarth has a great 2-player co-op mode.

So many good 2 players games. Playing at the same time.

Here's a few which were not listed befor.

Herzog
Blade Lords
girly block. - Suprisingly it works much better as a 2 player game.
Laydock 2
Bet Your life. - Even with the annoying slowdown. The fun factor is still good. I found it actually better as the original Super Smash tv.
Stracon - (up to 4 players. It looks not so good, but it actually can be quite a lot of fun !)

I´d also suggest F-1 Spirit, Super Runner and Super Laydock.

These games could be fun too for two players: Warroid, Twinbee (although it´s annoying if one player steals or change the color of your powerups), many sports games such as Konami´s Soccer or Konami´s Boxing and The Race (adapted from the Spectrum, but it´s a simple and nice racing game).