From G1 I will have to echo what so many others are saying and state Rescue at Midnight Castle. It would be so neat to see that episode redone and fleshed out (redone in honour of G1, not in G4 style). I can't remember any specific Tales episodes but I was in love with the series as a child! G3 stuff passed me by entirely, unfortunately.
As for G4... what a tough question! I really love Art of the Dress, Party of One, Lesson Zero, A Dog and Pony Show, and The Return of Harmony (the Trekkie in me can't resist!) all stick out to me, but I love all the episodes!