Best Games
This list of recommended board games is sorted by number of players and then differentiates between different audiences and interests. Of course most games can be played with more or less players but most games usually work especially well with a fixed number of players.
2 Player
I think that most good two player games are actually two player games only. Some board games now have a 2-player version of a game that is also available as a e.g. 2-4 player version.