Nirvana in Fire is The Masterpiece! For me, nothing can even compare to this drama, ever... And Mei Changsu is the best character, because he isn't presented like some really good guy, who seeks for his revenge trying not to harm the ones around him, but as someone willing to do all that is necessary to achieve his goal, to bring his plan to end. And all he did wasn't for his own good, but for his family, for Prince Qi and for all his comrades. That's why, no matter who he harmed I always understood him and loved him. He is my favorite character ever!
And you're right, rewatching this drama is even better than watching it for the first time, all the angst is tempered down and you have time to notice little details that bring new great feelings about the show <3