Don't forget to evaluate each boss based not only in terms of combat mechanics, but also if it provided a fitting end to the raid while taking into consideration other things like the boss design, the lore of the fight,etc.
My favourite last boss was C'Thun. I love everything that is old-god related so having a fight with one of them proved immensely entertaining. Not only that but C'Thun was the boss that took more time for my guild to kill at the time as he required tons of coordination and most people really needed to know what they were doing, it was not a fight that half the raid could carry the rest like many others before. Oh, and the tons of Resistance Nature potions wasted on the countless wipes, oh boy, I still remember those farming nights looking for ingredients to make all those potions for the next days. Good times.
Edit: Rend Blackhand was actually supposed to be just Blackhand, the current tier last boss.