Things like sylveon would probably be some of the most popular atm, but that also means there are about fifty of them. I don't think you can ever go wrong with stuff like the eevee-lutions if you're meaning to sell something, but the most loved options are always going to be pretty saturated.
Dragonite might be a consideration -- there are only really a couple that come up in a search and most of those stem from one single difficult-to-find pattern. I know where it is (FINALLY), but who couldn't benefit from more? Yamask also only seems to have like one single option, and might be fun to figure out, and Murkrow is very halloween-y.
Speaking personal favorites that don't seem to have a lot of love:
β’ Amphoros
β’ Plusle/Minun
β’ Aron (for as cute as it is, designs are almost nonexistent!?)
β’ Omanyte
β’ Skarmory
β’ Banette