Music is great if you have a thing for epic/heroic BGMs, it's probably the best BGM in that game. 10/10 and BiS for your Orchestrion.
Difficulty when relevant was much lower than T7S or even slightly lower than T9 (we cleared T13 faster than T9). 6/10.
Mechanics were not fantastic. Compared to A8S it looks like 5 generations below. 4/10?
Boss design is lazy, just a floating mini Bahamuth with his regular Dragon adds. 3/10. Shinryu and Niddhog look way more menacing in terms of design.
Most weapons look cheap (that axe), some were OK, but non-dyable (same for other old raids). High Allagan weapons were much better looking on most jobs, not speaking about Alex/Midas. 3/10.
Imho not the best final-tier boss of FFXIV, probably the 3rd one after A8S and maybe T9. Others to be discussed (A3S/A4S were much harder but their charisma/design is non-existant so they're out).
So it's probably not the best fight in the history of gaming.
Let's give him an extra chance with Ultimate mode.
(Sidenote: A8S would get 9.5, 10, 10, 9 and 9 with that rating).