I'm not precisely sure which would be the cutest, but personally, I've enjoyed May's companionship quite a lot. Unlike most other female partners that he has had, May actually had something going for her other than just travelling with Ash; while his task is to defeat gym leader and eventually face off against the champion, her goal is to participate in contests and win. Ash would train his Pokémon for battling, and so would May, but mostly so that they would be able to better their performance and put on a decent show.
His other companions did have their own goals, but if you ask me, they weren't as major as May's. Not only that, but I'm also quite a huge fan of May's characteristics in general; she's a happy girl, but won't always be too joyous. If you force this person to her maximum limit, then she will display hatred and whatnot. She's very human, if you ask me; most of us could relate to her.