The current ranking, IMHO (100+ games under my belt) is
Erebus > Unclean Beast > Sedna > Oak > Regulus > Rook = Torch Bearer > Queen of Thorns.
Erebus is pretty much unstoppable. If you are playing against a team with Erebus and you don't have one on your team, you're in trouble. He's been nerfed a few times, but they've just reduced his strengths. He simply doesn't have any weaknesses. Unclean Beast is as tough, but doesn't have the minions that Erebus has, and therefore a bit lower in tier. Sedna can't deal damage but can soak it up like no one else. After level 10, a proper Sedna can sit in combat forever. Oak has a bit less durability than Sedna, but has great abilities. Played well he is a match for anyone. Regulus has range and speed, as well as extremely long range attacks. Supporting a team, Regulus can use snipe to kill weak opponents and his mines are very powerful. But he is not durable and lacks in direct damage. A poor Regulus player is utterly worthless. Rook and Torch Bearer have obvious weaknesses that can be exploited. Rook is very slow. TB is very fragile. Both of them can be overmatched and killed quickly unless the player is very aware. The Queen, sadly, is just awful. She only has a few good skills and they're often overmatched by other skills. Her best skill, shield, is pretty much useless by level 10 when your opponents will be dealing more damage than it can deal with. Her direct damage skills are too weak against enemy demigods, although they are great against minions and creeps. But that's just not enough. Only a very skilled Queen coupled with a good team is viable.