I'll go with G1s here.
In no particular order:
1.
Skippity Doo: Bizarre name, but her colour scheme is gorgeous, & who can resist a rearing pony?
2.
TE Skyrocket: I've always loved her colours, but I can see how they might not be to everyone's taste.
3.
Scrumptious: That shade of green is so refreshing against her white body.
4.
Sunbeam: Beautiful colour scheme, but I know her pose isn't very popular. The other three unicorns in her set are so well-loved, the poor girl always seems to be left out. :<
5:
Sparkler: Another pretty unicorn who doesn't seem to get mentioned much.
6:
Magic Hat: Lovely design.
7:
Sweet Pop: There's something really appealing about that vibrant shade of blue.
8:
Cherries Jubilee: Monochrome ponies often tend to be overlooked, & her pose is one of the more awkward ones. But she's very pretty & those curls are snazzy.
9:
Sundance: Poor girl always got saddled with Megan, but she's such a cutie in that shy pose, & her simple design works really nicely.
10:
Sunlight: I've always had a soft spot for her. That subtle shade of blue/aqua looks great against her rainbow hair.
I could write an entire essay on the many G2 ponies that are overlooked, but that's a story for another day.