My favorite, and most played game is absolutely Arboretum.

It plays quick, it's small enough to stash in a bag, it's easy to teach, and it feels like it has endless play and variation. The game also lets me do one of my favorite things in a board game, which is let me go for high risk high reward "shoot the moon" strategies. Absolutely lovely bit of tension and backstabbing fun, but concentrated all into the reveals right at the end so it doesn't drag the entire experience. Great at every player count, especially at 2 players which is important given that most of my board gaming is just with my wife.

Plus you get to look at lovely art of trees while you playing (with the original edition at least, the newer one has worse art IMHO).