I'll be another voice saying Adventure Quest and Runescape suck. I'd rate each at a 1. Hobowars is fun for a bit-- I'd rate it a 4. Neopets is generally underrated because of its primary age base, but I'd give it at least a 7-- simply because there's so much to DO. It's not all about raising little pets and battling them against each other; there are tons of mini-games, quests, etc.
I'd also like to add a game: www.popomundo.com -- Popomundo, which I'd rate at a 9. You're just a regular guy/gal with a dream to become famous. It's a browser game wherein you can be a heavy metal guitarist, punk rock singer, anything to do with music-- in addition, you can be a member of the paparazzi, a thief, a cop, a mayor, a lawyer... there's tons to do.
Also, I'll add Aardwolf. It's a MUD with a ton to do, but I'm too lazy to give a proper description at the moment, so nyeah. I'd rate it 8. www.aardwolf.com
Torn City is another I didn't see listed, but I'd rate it a 3. www.torncity.com
Marapets is a very lame, very sad Neopets knockoff. I rate it a 2.
A3 is fairly lame and I got bored quickly. I'd rate it a 4. You have to donate to get anything cool, too.
Aegis is an alright if slightly boring resource management game. (I love resource management games). I quit this one the same day I started it. I'd rate it a 3.