Below is a list of studios I've worked for which I've had a good experiences with & that I feel were professionally run.

Next Door Studios

Mercanary Pictures

Blue Bird Films

Kick Ass Pictures

Bangbros

Jules Jordan Video

Naughty America

Realty Kings

West Coast Productions

Black Ice

MBIT Productions

For a (nearly) complete Wikipedia list of studios CLICK HERE.

Keep in mind, the "top" studios are the following:

Vivid

Wicked Pictures

Adam & Eve

Digital Playground