Ideal JAV actors are Jo Oshima and Ken Pierre. Both have great, clear voices. Don't stutter. Both are relatively handsome and can perform. Both also know that once play starts they should shut up.
Worst of known names: Abe-chan when he just plays himself. Guy slurs a lot, injects weird badly pronounced English, and can't seem to shut up during sex. I'm sure he's fun as hell to hang out with in real life, but JAV movies should be about the actress, not the guy doing her. Conversely, he plays a fantastic doctor, but still should see a speech therapist to fix his slur. It's not super noticeable, but once you notice it, it's hard to un-hear it.