A high wing, flat bottom airfoil, preferably 60 sized plane, with moderate, but not excessive dihedral.
My first trainer was a Great Planes PT-40. I followed the manufacturers recommendations and built it with the "trainer wing" which has lots of dihedral. It was a very stable plane, almost too stable. At times I felt like I was fighting with the plane just to get it to go where I wanted it to go!
After a 15 year hiatus from the hobby, I'm building a PT-60 with the lower dihedral sport wing.
Really, any of the time tested trainers out there will serve the purpose well.