My vote is the Sig Kadet MkII. My Dad has had one for 27 years and it has trained about 8 pilots! Lots of fixes over the years, but she's a little sportier that the LT-40, a little more challenging to build though. The kit can be built as 3 or 4 channel, but you'll definately want to add the ailerons.

We have a high performance NACA 0015 airfoil wing that we can swap out with the flat bottom wing and she fly's like a sport plane just slower and a little more stable.

By the way, my Dad's is done in the original Orange and Black. She still looks great.