Guldan was nice but line of sight mechanics suck so jaina and azshara are not that fun.
But to be fair most endbosses since Ulduar were pretty good. With the occasional exception like Kiljeaden in ToS, What an atrocious boss that was. A fitting garbage endboss for a garbage raid.
I personally would say: Lei-Shen > Ragnaros (FL) > Lich King