To get back to "What makes a good actor?", for me it's pretty simple: If I fully believe you are this person who you are pretending to be on screen -- even with me knowing you're not -- then you're acting successfully. And you're an even better actor if, as this person, I can see the world through your eyes and resonate with you and you can convey your emotions and thoughts to me so that I feel them and understand them.
There are qualifications to that. (For example, if I know you are playing REALLY against type for who you are and what you've done in the past, that increases my estimation. If I know you are just being yourself, then that's not as much "acting" per se. Or if you have a terrible script, it's harder for you to seem like a real person no matter how good you are.)