I think Richard Bach's point, above, is very true.
My favourites would be Slingsby T67, Grumman AA5A, pretty well any of the Robins.
Why? Side-by-side seating is advantageous for the instructor but more importantly pleasant for the student, they are nice to handle (I would never have taken a PA28 or C152 up just to pole it round the circuit), good visibility of one's environment, and while pilots of only poor-to-moderate ability can get them safely up and down again, they reward excellent skills. Oh, and they look like real aeroplanes (which makes one feel like a real pilot).
I think flying schools very much under-estimate the things that genuinely could keep more students coming back for more.