As much as my rose-colored glasses want me to laud Burning Crusade as the best period in World of Warcraft's history, I don't think that statement is true.
A big part of why Burning Crusade has a special place in my memory is because so many of the experiences I had that expansion were new ones. I hit the level cap, ran dungeons that couldn't be beat by outleveling them, lead a guild, raided, made vides and so much more all for the first time. It's not surprising that a period of intense discovery would engender unfair nostalgia and acclaim.
But in reality Burning Crusade was still deeply flawed. The improvements it made over Vanilla were considerable, but it still bore a lot of baggage from what came before. There were classes and specs that were unequivocally superior while others were worthless. There were design issues with quest chains and bottlenecks. Itemization was still awkward and confusing. Burning Crusade was better than what came before, but it wasn't the best that WoW could be.
However, I think Burning Crusade is unique in that it is the only expansion that was arguably a universal improvement. There are a couple of contentious topics, such as raid sizes and badges, but even there Burning Crusade had fewer of those than the expansions that followed.
Overall I think Burning Crusade was the second best expansion, and I'll be discussing the first shortly.