okay but uh not anthromorphic pokemon... idk if my followers would like it if I drew like that :o

I'd say start off with a Pokemon you like, but maybe an uncommon one. Trying to think of other tips...

I agree with the person above me: Uncommon pokemon tend to be more original, but you can really do any pokemon that you want.

Here are also a few other things to consider:
What universe will your pokemon be in? Will it be in the main series, the PMD series, the Ranger series, or perhaps the Conquest title?

Take your time on the biography of the character and make sure it's not too unrealistic or cliche, and you don't want it to be vague, either (unless your character's bio is coming up in a fanfic or a comic, and you don't want to spoil the details.).

When deciding the color scheme for a character, go easy on the colors (limit VERY saturated colors, especially neon), and try to go with a color scheme that goes together well.

If you're doing a character that has any sort of disorder, please do your research, unless you made up the disorder yourself.

Sorry this was so long....