This obscure MMO called Champions Online. Superhero MMO, great customization, but so little content. You could change your costume independent of gear, and you didn’t have to unlock the vast majority of the pieces. You could also completely customize your build. You could have a Fire mage mixed with a Shadow priest, it was great.
Downside, though, the max level is only 40, and has been 40 since launch. It was a small little thing I could cherish while I liked it, with like 5k players at most. It also helped my young annoying butt learn MMO etiquette, as well as learning basic controls.