Nanjie used to be, because it's the French school's main hangout, and many memories. But the alcohol is absolutely AWFUL, it's like a blend of piss and bleach.
Now I think Club 13 is still good, because of the concerts, good alcohol served generously, but since the rock scene is slowly fading, there isn't that ambiance there used to be.
Maggie's is a good balance of everything, but it's a bit expensive.

The first bar I ever went to to in Beijing 2 years ago was Saddle Cantina, and I still like to visit this bar. Its nothing special but I like the big drinks, the atmosphere and the outdoor deck.

Its my go to bar.

I also like the Den, mainly cause of all the characters that walk in there at very irregular hours

Oh yeah there's Lush, next to the wudaokou subway station. They've got good food, good drinks (100 yuan for 1.5 liter cocktails), and you can access FB and all that there.
They have different events depending on the day, like quizz night, and it's mostly a grownup student hangout, so the atmosphere is always friendly.

Changes up all the time. When I first came it was Paddies alot, then siif, then the box, then fubar...etc.