[the phantom]

1 Sauron- most agree on this
2 Gandalf- same thing
3&4 Saruman and Balrog- I'm considering Saruman before his downfall
5 Elrond- great healer, ultimate lore master, excellent forsight, very wise, controls a river, has Angelic ancestry
6 Galadriel- learned much over her long years, very experienced, resolute and mentally strong

Then I start having trouble. Here's some candidates for the next slots-

Radagast- He was probably somewhat hindered from using his full power like Gandalf, but how powerful was he really? Saruman sure didn't think very highly of him.
Glorfindel- He was a mighty elf-lord among those that rode forth from Rivendell because, as I recall the book saying, he had the power to ride openly against the nine.
Witch King- We saw and heard enough of him in the books to know he was a force to be reckoned with.
Celeborn- He was the ruler of a great realm and was said to be a great giver of gifts and also wise.
Cirdan- He was a very ancient and wise elf with great forsight, though we don't see him doing much (like Celeborn and Radagast).
Fangorn- He was very strong and knew a lot but who knows how he'd measure up against the others?
Shelob- She was a very nasty creature, then again a hobbit took care of her.
Smaug- He's probably my pick for number 7. If it weren't for that little weak spot in his armor who knows?

I'm sure I'm leaving some worthy beings off, but I don't have the time to list everyone.