Competitive (everyone against everyone)
The Castles of Burgundy - http://boardgamegeek.com/boardgame/84876/castles-burgundy
	Not played this one myself, but heard amazing things.

Hey, That's My Fish! - http://boardgamegeek.com/boardgame/8203/hey-s-my-fish
	A great light game and easy to transport.

1 vs. many
Letters from Whitechapel - http://boardgamegeek.com/boardgame/59959/letters-whitechapel
	An amazing game, that plays well at 2 to 6. With 2 it is straight head to head, as you add more it becomes 1 vs. the rest.

Most co-op games will be great, some good choices there:
Pandemic - http://boardgamegeek.com/boardgame/30549/pandemic
Forbidden Island - http://boardgamegeek.com/boardgame/65244/forbidden-island

The traditional card games are also always good
Magic: The Gathering
Android: Netrunner http://boardgamegeek.com/boardgame/124742/android-netrunner
Seasons: http://boardgamegeek.com/boardgame/108745/seasons
	This is a card game with more of a box/board.

Quarriors: http://boardgamegeek.com/boardgame/91536/quarriors
	The original plays amazing at two.

If you up for a SUPER HEAVY, ALL DAY, COMPLEX simulation
Twilight Struggle = http://boardgamegeek.com/boardgame/12333/twilight-struggle


Also see:
- http://www.reddit.com/r/boardgames/wiki/wsig/two_player
- https://www.youtube.com/watch?v=k3J2p6vc_5A
- https://www.youtube.com/watch?v=lnZM2oDBqrg