All Good Things - The perfect TNG finale
Tapestry - This was when Q didn't come across as evil or mischivous, but more along the lines of a teacher
Death Wish - Great character study of Q as a whole.
Honorable mention to True Q, just because I find that episode really underrated. People say it was boring, but I didn't think so. I thought it did a great job shining light on the Q as a whole, before Death Wish came along and expanded on that.