I like mythological creatures. Anivia, Ahri, Cassiopeia, Galio, Nami.
I also like fantasy characters like Xerath, Karthus, Fiddlesticks, Thresh, Maokai, Hecarim, Morgana, Blitzcrank, Nocturne.
Then there's humans which look "exotic", particularly like mages or priests would look like in MMORPGs. For example, Janna, Lulu, Malzahar, Brand, Ryze, Veigar, Sona, Lissandra.
Then there's humanoid champs with specific styles like Ezreal, Jinx, Jhin, Graves.
Then there's modified humanoids like Mundo, Sion, Yorick
After that, they start becoming neutral and disliked champions, but to varying degrees. From best (neutral) to worst:
- Humanoids which aren't to my taste or generic humanoids like Sivir, Vayne, Fiora.
- Badly proportioned humans or really buff humans like Garen, Illaoi, Gragas, Braum.
- Odd ugly stuff like Teemo, Urgot and Void champions.
That said, it always varies based on the style of each individual champion. For example, while I love mythological creatures, I hate Aurelion Sol's design. He looks so bad. I think it has to do with Riot wanting to establish their own vision/interpretations of whatever they make. In trying to do that though, you sometimes make horrible stuff just to make it unique. A dragon, whether an eastern or western version shouldn't look anything like Aurelion Sol.