Can I place a vote for the humble Cessna 150/152?
Much less interesting to fly than Tiger Moth, Chipmunk, Bulldog or Tutor. Not really intended as a military trainer.
But they are simple, robust, stand up to endless abuse and being flown overloaded; easy to fly adequately but reward good and accurate flying; stable enough for instrument flying; can be used in aerobat form for (very) basic aerobatics and upset training.
They've probably trained more people to fly than anything else and are so hard to find a replacement for in general aviation that they command ridiculous prices.