I’ve already answered this question, but people keep asking!  The cats in Sailor Mew aren’t all based on specific breeds, but here are some people have suggested (with photos for comparison!)

Sailor Jupiter:

I originally just wanted her to be a giant fluffy cat, because she is canonically the biggest (as her planet is) and her powers involve electricity, so she would probably be constantly poofy.  A lot of people suggested Maine Coons.  They are huge, fluffy, and protective.  Seems a good match to me.

Sailor Mercury:

I had a very definite breed in mind for her. Or rather, a mix.  She’s a Scottish Fold/Munchkin.  Munchkins have very short legs, like daschunds:

And Scottish Folds have folded ears and perpetually surprised/heartbreakingly adorable expressions:

Combined, the attributes of these two breeds make them the most potently kawaii cats in the universe.  And they really do have a habit of standing on their hind legs!  (Here’s a video. You know you want to see it.)

Sailor Moon:

Usagi is a Japanese Bobtail. It’s a popular Japanese breed steeped in folklore.  Bobtails’ unique look inspired “beckoning cats” used to bring good fortune to shops in Japan.  This breed’s cute stubby tails also makes them look like a bunny!

Sailor venus:

I didn’t have a breed in mind for her. I just wanted her to be very elegant and vain and playful.  She’s proud of her giant fluffy tail and uses it to get attention.  A Turkish Angora seems a good match for her.  They are amicable cats known for their exceptionally luxurious fur and floofy tails.

Sailor Mars:

I had the most difficulty finding a specific breed for her. I’m still not sure there’s one that really suits my idea. She’s a little fluffy, but not as fluffy as Jupiter or Venus. And she’s kind of a skittish cat who prefers hunting things than sitting in someone’s lap.  Unfortunately, the only cat photos I can find that look like what I imagine don’t have a listed breed. (They are mostly feral cats.)