I am fortunate to instruct at a club that has a varied fleet.
DA20A1, DA20C1, PA28, C172, DA40, T67M and L21B (SuperCub).
They are all great in their way, but I think the best trainers are the DA20A1 Katanas. Their meagre 80 HP means that the students need to get the climb speed exactly right, and the ball exactly in the middle in order to get any reasonable climb rate. The bubble canopy gives a fantastic all round view, the stall is benign (although the nose does not drop), they spin and recover nicely and they are not too easy to land. They have a CS prop, which prepares students for more advanced aircraft. Economical too, at 15LPH.
I love flying the Cub, and doing Tailwheel conversions in it, but I would not recommend it for ab initio training. The tandem seating makes it harder, and with a big student it is impossible for the instructor to see the instruments. Also not entirely happy with the student being the only person with access to the mixture and radio controls.
The T67 is also a great training aircraft, but a bit too easy to land. Great for aerobatics though!