I think Dragon Saga and LaTale offer the most customization without paying though if you don't mind dishing out a few bucks then MapleStory I think has the largest selection of cosmetic stuff. I'd say maybe try one of those.

Pretty much all of the side scrollers are enjoyable if you don't take them too seriously, but they all kinda have their ups and downs so you may need to try out a few before you find one you like. To be honest, a lot of the really fun ones closed down like Dragon Gem and Wonderking or are foreign such as Tenvi and Everplanet.