7-wonders.This game is easy to learn tough to master. Its got a great strategy element and plays fast. The biggest win for this game is its great with 2-7 players. Good for die hard and casual types. This game is just pure win.
This board game is great for 4 player. The game is themed really well to the series. One knock I have is the game is a bit too do your own thing and players do not engage each other at all. For people who like competitive but not direct PVP games they will love it.
These are my two favorite games at the moment. Both look intimidating with all the card types (especially Firefly—there's 13 decks of cards!) but go very smoothly once you get used to turn flow.
One I just got for Christmas that's likely to join the top is Forbidden Island. It's the same game as Pandemic, with all players having different roles and every player's turn triggering new threats, except the players are teaming up to collect treasures and escape before the island sinks out from under you. It's simpler and faster than Pandemic (designed for a younger audience), but also much more dynamic as you are constantly losing parts of the island to flooding. You can also go to BoardGameGeek.com and get new tile layouts, some of which are brutally hard.
I really like Race for the Galaxy, too. You build up your civilization by settling planets, developing tech or institutions, or producing or selling goods. You have to choose which action you're going to take each round, but you also get to do the actions the other players choose that same round, so reading the other players is important.
There's also San Juan, which the same game except simpler and players choose actions in order instead of at the same time.