In a sense, each generation did something "right". I am talking physical pony details here. If you are to gather a "perfect" using all four*, what would you take?
I would:
g1: muzzle and back legs, unicorn
G2: crystalline eyeshine & leg lenght
G3:regular - gorgeous gradiented eyes & front legs
3.5 - rich, thick mane & cheecks
G4: - wings, non-hollow legs, unipegasi existing
...Oh, and ears of
Magic Meadow Pony - they are, too, Hasbro, and I am allowed to cheat
*"g5" is 4.75 and I am rolling down that hill