I wouldnt say demon but my favorite transformation or chakra beast/biju is
The 9-Tailed Fox !!
posted over a year ago
12 years ago, a nine-tailed fox suddenly appeared. Its tails lashed out, sending tidal waves crashing to the shore. The ninja rose up to defend their villages. One brave shinobi sacrificed his life to seal the fox in a human body. This man was known as... the Fourth Hokage. I have that intro memorised stone cold.
i think my favourite demon is the three tailed demon that kind of looks like a giant turtle that dosen't have much power until it nose it's game time :)
Naruto in Tails Mode makes him look creepy and ugly; Sasuke in Curse Mark Mode looks epic. So it's obviously Sasuke. (And did I mention that Sasuke could fly?)