That's a brilliant question to ask!
My best experiences have been with tutors/teachers who were friendly, focused, and taught me in a systematic way. Even in conversation classes, it's important for me to get feedback, such as Skype messages for error corrections or Google Docs with new words and phrases. This might seem obvious, but the best teachers also had an excellent grasp themselves of everything they were teaching me.
When learning grammar topics, it is very important to practice them, so I love getting exercises.
Apart from these, a good teacher/tutor for me needs to be approachable with a sense of humour and needs to be patient yet encouraging. Empathy is important.
My not so good experiences included tutors who just read words from a book to me, who were uncertain about the topic they were trying to teach me, and in one case a person who explained the grammar in the target language at a very fast pace, which was too difficult for my level.