It depends, do you mean bands that came just to play in China, or foreigners that start bands in China?

I was in one of the latter. I'd say we were one of the least bad, compared to those "experimental" bands (hipsters pretending to be creative geniuses while randomly slapping various instruments)

One of the most skilled I've seen was "Maggie Who", along with some other bands whose name I forgot.

The worst were, as I said, experimental and indie bands, and this one band of 4 male teachers in an international school, in their 50's, suffering from a mid-life crisis and trying to be young again by playing in a rock band (they were awful but took it seriously).

But if you're referring to bands that came from abroad to play in China, well, there were quite a few famous bands, so I guess they weren't bad. 

Almost every Filipino band that play in bars, clubs, parties, etc...  Most of them are truly horrendous and when they choose to brutally murder a favored song of mine, it makes me just want to walk up and slap them all.

The only exception to this, that I have seen, is the Filipino band that plays at Alice Bar in Dalian.  They were actually pretty good. 

Black Rebel Motorcycle Club came to China last year and were amazing live. They made all the other Chinese bands before them sound amateur. In Beijing there are lots of foreigner bands, I cant even keep track anymore but I've always liked the Amazing Insurance Salesmen. They're a solid, talented band.