Totally subjective. But for me my Open Water Instructor stands out in my mind. We did full classroom (no slides/video/dvd - we were in the phillipines in a beach shack) and he took the time to explain everything really well. He was firm with us in the water, when we gave an "OK" it had to be a firm strong "OK", no floppy hand "sorta OK sign. I carry that with me today. The skills were easy, neither of us had anxiety, having had a pool in the backyard, both have worn mask/snorkles/fins on a regular basis and skin dove in various oceans on previous vacations.
I left Advanced OW in tears, my husband made me go back. The instructors (here) were not good, they treated advanced as an underwater tour, rushed us, I had no drysuit training prior to my first dive, just threw me in, I remember entering thru the "cut" on a stormy october day, the cut choked with big logs and lots of surge. And I was made to feel dumb for asking questions.
Luckily I took Rescue and had another great instructor, patient beyond his years and answered my million questions about all things diving, which lead me to being his DM student, and so on. If it weren't for Doug I probably would have not continued. His attitude (lack of elitism I encountered in advanced) and his patience and sense of humour. I DM'd for a year for him.
You can ask 100 divers what makes a good instructor, and get 100 different answers.