I mean, they all suck. But you're probably best off sticking with one of the bigger carriers, like China Unicom. The smaller companies are cheaper, but it seems like they are more likely to throttle bandwidth and just be generally unreliable (in my limited experience).
Your Internet speed will be determined more by the building you live in - hope you live in a newer one, cos the older buildings don't support high-speed lines. At least, that's what they told us at China Unicom when I tried to get a faster connection in my previous apartment.