I think the best usernames are simple, easy to read, and don't have any silly numbers or underscores or decorative xXx's. I prefer shorter names over long ones, but I've seen some awesome longer usernames that really show character.
My first username, I grew to hate. It was my IRL nickname that only my dad ever called me, and I didn't even like it that much. Earlier this year I grew sick enough of it to be bothered to change it, so I picked out a long, flowery character name. I didn't like having a long name, idk why but it really bugs me. So I changed it to the first thing I could think of that wasn't taken, tinyko, the name of one of my neopets of all things. It was just a placeholder until I found something I really liked.
And yesterday, I did! I was excited when I typed it into the gaia search and no results popped up. The name was also free on deviantart, gmail, and wordpress (and maybe everywhere omg I should sign up for everything). Basically, I combined my favorite drink with fae, which means fairy, which was fitting since I love fantasy, mythology, and folk tales. Its short, sounds cute, and actually fits me as a person.