I love G1 the most, because there's so many different colors of ponies. G3 would be second since they have many colorful ponies.
G4 is not even on my list, there's no variety of colors just the same 6 ponies over and over and over again.
G2 are cute and they didn't make alot of them but what ever they were made of degrades and discolors faster than G1.