My preferences are kinda all over the place. Generally I like Pokemon that have more subtle colouring in colours I like - purple, blue, black, etc - so that's most Ghost and Ice types. I especially like Ghosts because they're such a varied type; they take inspiration from all kinds of things and it's a really nice blend of cultures that's not quite so obnoxiously obvious as Legendaries etc. I like Pokemon based on animals where the real-life inspiration isn't so immediately obvious, because it suggests to me that more thought has been put into their design.
I also like most inanimate object Pokemon, even though their inspiration is usually pretty obvious. I've said it before, and I'll say it again: anyone can make a Pokemon based off of a real-life animal. It takes creativity to make something based off an ordinary object and make it actually look like it's alive. It doesn't always work, of course, but then you get things like Sinistea and Klefki and it's great. The names of Pokemon like this are often quite amusing as well. I occasionally like weird Pokemon if their designs stick out enough to me too. I'm very fond of Celesteela, for example...it's a skyscraper rocket with arms and it reminds me of that one Zelda boss with disembodied hands that is in literally every goddamn 3D Zelda game from Ocarina of Time onwards, and even a couple of the 2D ones. It's appealing for some weird reason. I also quite like Naganadel for being a purple-coloured Waspmon, and Eternatusis is fantastic.
Sometimes the ones that look less like Pokemon are actually the best ones I think, because they somehow fit the theme whilst not fitting the theme at all. I think I like these ones because they're not excessively detailed (I mean, Eternatus looks messy in a lot of artwork, but that's because it's curled up on itself and it's transparent - stretched out it's not much more complex than Rayquaza) and the best Pokemon designs aren't ones that have so much detail there isn't a clear focus point for the eye. Or, in other words, the ones that don't look like they were designed by Tetsuya Nomura.
Also if I see an unintended reference in a Pokemon I'm immediately drawn to it. I mean, Solgaleo is obviously the
White Tigerzord.
However, all my preferences are annulled if the Pokemon in question is a) primarily brown and/or orange, because those colours are vile, b) part-fighting, because that is the absolute worst type, or c) causes me enough aggravation in-game through status effects or repeated random encounters that I get fed up with seeing it.