I'm surprised to see The Imprisoned winning by so much. The Thunder Dragon's Lightning Round is a good way to decide who my least favorite boss is, but I honestly have to say that mine would be Koloktos. I am simply baffled that he's apparently considered the best boss in the game, and while Tentalus does look silly and came out of nowhere, I actually really enjoyed the battle, and while it got annoying, it might actually be one of my favorite bosses in the game, but that's just because I enjoyed the Sandship level a lot.
The Imprisoned is actually better than fighting Ghirahim three times. The Imprisoned at least changed its ways to climbing, and I love the catapult sequence in the third battle, even if I missed the first time. With Ghirahim, the second battle is really just an exaggerated version of the first battle, and the first phase of the third battle was remarkably easy, but probably my favorite battle with him because of how the second phase was.
Moldarach was remarkably easy when I fought it the first time. It makes me rather surprised that I don't seem to have as easy a time fighting it when I play the TDLR.