This is a complex question, with a multifaceted answer.
Consider the following criteria (in no particular order):
1. build quality/construction techniques
2. sound reduction
3. attachment method (more important for rifle)
4. serviceability (critical for 22LR)
5. boostered vs unboostered (critical for most pistols)
My personal opinion on bad qualities: Some manufacturers are running with really out dated cans. Others are built very poorly and are sold at top flight prices. Others wobble when mounted using fast-attach methods. Some 22LR cans dont open for service and are made of aluminum, rendering them impossible to clean effectively. Some manufacturers are happy to sell you a pistol can that lacks a booster and will turn a reliable pistol into a jam-o-matic. Others just sell a loud ass can and call it gravy.
My personal opinion on good qualities: Rifle cans that are welded together for strenght. Pistol cans that have boosters for reliability. 22 cans that are made of steel and come apart for cleaning. Fast-attach methods that lock-up right as a thread-mount and don't unscrew. Sound reduction is secondary to reliability, and longevity.
My personal likes: AAC and SWR. My personal dislikes: Gemtech.
head on over to silencertalk.com and silencerresearch.com and do some research. Take everything you hear with a grain of salt.
-T