Carl Goldberg Eaglet 50
If you can get your hands on one, this is a great little trainer. Stable, predictable and forgiving, this trainer offered many great flights.
This only came in a kit, but was quite easy to build in a few weekends. I built one in the 80's when I was a kid. I guess I've never really grown up, because I'm building another one right now. I intend to teach my son how to fly using the exact same model.
It will even have the period correct engine I put on it way back when: The O.S. Max 25FP.