2 players: 7 Wonders Duel, Hive, Schotten Totten, Sobek: 2 Players, Cribbage.
4 players, also great for 2 and 3 players: Arboretum, Azul, Calico, Gizmos, Happy Little Dinosaurs (good for younger kids), Photosynthesis, Shifting Stones, Sprawlopolis, That's Pretty Clever!, War Chest.
My Girlfriend and I play a lot of 2 player games. 7 Wonders Duel and Schotten Totten are our personal favorites. Calico and Arboretum are also a lot of fun.