Definitely the Sweetheart Sisters (especially Glittery) and Colorswirl ponies! There are so many good gimmicks for G1 though! I have like a whole list of the ones I'd like to collect, although I actually might need to add more to it, but those are definitely the big two.
I am a huge fan of G2 ponies since that was my first gen, and Sweetheart Sisters are like a blockier, more mature version of them (with those fabulous TaF-style glitter marks!) - which to me is perfect! You'll find a mix of dynamic blocky shapes in with organic shapes in my own art style, and the Sweetheart Sisters are similar to that for me. The colorswirl ponies are just hecka unique, I've always found them fascinating and the fact that there's only three designs of them makes them even more special.
In both cases it was a huge deal for me when I found out these ponies were real, too haha.
Dude honestly YES, HUGE YES, but this is a G1 thread haha