[obloquy]

Sauron is probably the most powerful of the maia--all maia are not equal.

No, they're not all equal, but there's no reason to believe Sauron was the most powerful. He outranked all of Morgoth's other Maiarin servants, but that only means he was the most powerful Maia to rebel.

Morgoth was the most powerful of the Valar because he used all of his thougt and power to increase his power, while he was also the most powerful to begin with.

Melkor did not increase his power. He was his most powerful in his beginning and went downhill after that.

and somewhere I think Tolkien even said "Sauron at the end of the third age could be considered as powerful as Morgoth at the end of the first" or something like that (in a letter or note, not text).

It's in Myths Transformed from HoMe X. But if you read the context, you'll see that what it means is not that Sauron had necessarily raised himself to Morgoth's level, rather that Morgoth had become so weakened.