It's hard for me to say anything useful because my brains like "mmm yesss pony symbols, gooood"
I love celestial symbols like Moondancer, Glory, Starshine, Night Glider . . .
Animals like Dancing Butterflies and Sea Swirl . . .
Plants like Sugarberry, G1 Minty, G3 Cherry Blossom, Keen Bean . . .
Food like Fizzy, Pretty Pop, Gingerbread . . .
And just random stuff, like Whizzer, Parasol, Dewdrop Dazzle . . .
For least favorites, I'm not big on the Sundae Best ponies or Pocket ponies (but I don't mind the G1 Princess medallions) and I don't like how almost all G2s have a single-color symbol. (I know there are a couple exceptions, like Light Heart.) Just seems like a weird decision.