That's a loaded question around here... most of us have our favorite knock-a-bout planes.

I would venture to suggest something in the .60 - .90 range in size.

This is large enough to fly well and yet not be problematic to transport.

Yet the engine size is small enough not to cost you an arm and a leg in glow fuel...

MiniFlyer's suggested planes are good, as is the Pulse and the Skybolt...

I like my Skybolt so much however that it's one I avoid dragging out all the time, simply because I don't want to risk damaging it.
It's too pretty to beat up.