I'll submit this for ridicule. I'd like to know what other people think.
My Ratings
Basically, we've got Max300. And there are a bunch of songs that are about as hard. Like Paranoia Survivor, some versions of Pluto, and Maxx Unlimited. These are songs that are so much harder than HVAM or Captain Jack that you can't call them nines. So we call them 10s.
And then there was PSMO. I think that was the first solid 11. Since then we've got more songs like Paranoia Respect that are so much harder than the 10s. You think of PSMO, and then you play Max300, and there's big difference. (I put LoM as somewhere in between the two.)
Level 12 I know is going to be controversial. But you have to admit that Dead End (Groove Radar Special), FaxX, and even Max300 (SMMM) are much harder than Paranoia Respect. It's just a matter how much. Is it worth a whole tier? I dunno. I'm positive 12s do exist though. Max300 to FaxX isn't even close to a fair comparison.
Remember - this is how hard a song is to clear, not how hard it is to AAA. Sometimes, like with Maxx Unlimited or LoM, the two achievements are drastically different. And forget about ITG's rating system. It's different. So Deep would've been a 10 on their system and Charlene is a 10 on my system, about even with Maxx Unlimited. (Charlene is officially rated 11.) ITG also had mines, hands, and doesn't have enough 'data points' to make comparisons with. (Yes, Vertex^2 > all. But that doesn't help me define what a 12 is.)