Twilight Imperiumif you can find a group of 6-8 masochistic friends that are willing to participate in a 12-16 hour game. My gaming group gets together and plays once every four months. It's a great mix of strategy, diplomacy, combat, trade, and resource management.
Combat is much like axis and allies. It is well balanced, although high end ships kind of dominate late game. The good thing is that the way fleet logistics work in the game, you can never build a fleet large enough to just run through the galaxy without checks and balances. You can have very large fleets for sure, but you have to be fairly strategic in their usage.
The game's diplomacy system is pretty unique. Creating alliances and trade agreements all happens between the players dynamically over the duration of the game. The built in diplomatic feature is a "galactic senate" of sorts where players get together and vote on various rules and agendas once per turn which can have significant impacts on the game.
Not for everyone because of the time investment, but one of the deeper board games I have ever played.