Quicksilver based on feats, direct comparisons, and the hard limits placed on the CW speedsters.
For example, Zoom stood there and caught a handful of bullets from the CCPD unloading at him for a few seconds.
Quicksilver on the other hand, watched bullets crawl across a room for a few minutes while he dicked around and told the laws of physics to go f*ck themselves(by pushing bullets out of their flight path for instance, or tapping people and having so much kinetic force people go flying with a slight tap). And that was before switching into second gear.