This is the #1 age-old question in R/C. What size engine are you planning on using? As stated above there are lot's of good trainers out there. Overall, you want something with;

-high wing
-flat bottom airfoil
-nose wheel
-ailerons
-not too fragile (although the Kadet Senior flys the best it's not too crash resistant)

I learned on a 3 channel Kadet MK I and it was a great flying tough little bird but there have been advancements in the last 30+ years

Yak