I can't vouch for any of the other suggestions as I haven't personally heard them but I continually hear good things about them. (rather I've not come across any genuine beefs regarding the RE subs, the IA subs, SI subs, etc)
Alpine Type Rs I've heard and liked. Can get very loud and sound good doing it.
I really, really like my Diamond Audio M6. Stuff it in a good box, feed it 600 watts and you'll like the results, trust me. The newest version, the D6, isn't terribly different so I would recommend it as well.