Without a shadow of doubt the best aircraft to learn to fly on is the humble glider because:
a. It won't turn if you fail to coordinate the rudder with the aileron.
b. You are always on minimum fuel.
and
c. The penalty for misjudging the approach is always a solid obstacle.
I started on these as a teenager then did a short PPL on Cessna 150's. Horrible thing but the reward was many happy hours tugging on tail draggers. Again, if your basic stick and rudder skills are not up to it you will have a problem. E.g. tugging a student on a turbulent summers day.
Next stop for me was the JP3 where any bad techniques acquired in the aforementioned were brutally displayed and required to un learned and then re learned.
The skills I learned on that little pig undoubtedly saved by bacon more than once may years later in charter operations on the B757 and B767300ER.
So, for my money, the JP3 has to be the best - there was simply nowhere to hide and, frankly, the more terse, and cynical, the QFI the better!
Regarding the light twins and the civil IR that's hard to say as it was a quick 8 hours learning the numbers on something horrible like a Piper Seminole then parting with a fat cheque to CAAFU.
To wrap up, even in today's world I see no excuse for not equipping a pilot with a thoroughly sound set of stick and rudder skills and a healthy fear of terra firma and a glider is a great way to learn your a b c.
P.S. When this sodding lock in is over I'm going to get off Pprune and take a trip to my local gliding club to see if I can still remember it!