As yet I've found nothing that says Lucifer was the mightiest. I've found references that speak about his power and how perfect he was
Ezekiel 28 'You were the model of perfection, full of wisdom and perfect in beauty'
You were anointed as a guardian cherub, for so I ordained you. You were on the holy mount of God;
In man's place, Satan became the temporary ruler of the world. The devil's temptation of Yeshua in the wilderness clearly shows this:
LUKE 4:5 Then the devil, taking Him up on a high mountain, showed Him all the kingdoms of the world in a moment of time. 6 And the devil said to Him, "All this authority I will give You, and their glory; for this has been delivered to me, and I give it to whomever I wish." (NKJV)
There's more info at
http://www.aristotle.net/~bhuie/satan.htm
The article seems to show that Satan ruled over the world and was more powerful than all the other angels. Unfortunately the Bible is kind of hard to interpret.
Revelations also has some useful passages, but I don't think anyone will ever find a passage that directly refers to Lucifer as being the mightiest. But i'm interested now, so i might look for a while longer.
Since there seems to be no passage about this (or if there is, it's obscure and hard to find) why does everyone assume Lucifer is the mightiest? What led you to this conclusion?