Almost was going to say Ifrit, but decided on Titan.

Ifrit's theme is pretty generic foreboding boss music compared to everything else that would come after, but since he was the 1st, I'll give him a pass. Titan's last phase music is just my least favorite genre.

It doesn't help that I will always remember the trial as not playing nicely with a lot of players because of the early growing pains of the game with lag/netcode/ping/RNGesus/whatever you blame it on.