1) 7 Wonders
While I have built up a fair bit of variety in my board game collection, 7 Wonders is still the game I end up wanting to play in every situation. Scales well from 3-7 players and it supports a number of playing styles. It's basically card drafting a la MTG, but with room for a lot more strategy and interplay. Can't see myself getting tired of it, and I can't recommend it enough. I would stay away from the expansions/map packs, with the exception being Duels (though it's more of a standalone game).