From out of all that you have attempted.
Currently 15/16 (only done about 5 pulls on Sha of Fear), but so far my favorites have probably been Elegon overall. Was a very tense fight the entire time and was the greatest yells in achievement when we finally got him down. Also love the Attenuation mechanic on Imperial Vizier, felt like I was playing some retro space game where I had to dodge stuff flying at my ship. Honorable mention to Spirit Kings as well, just for how busy the fight is, very entertaining.