|Image Courtesy of TheDDGuides|
As far as least favorite expansion, even if Arms DPS was godlike Cataclysm earns this (dis)honor. Not wrapping up Neptulaons story, or how disappointed I was in Thrall stepping down when the Horde needed him most are just a few of the things that rubbed me the wrong way. The fact that we killed potentially the biggest threat to Azeroth well before his time, unless Sargeras will finally not fail at coming to Azeroth in his full power and glory.
To sum it up I think all of the expansions have had their bright sports as far as game play, quality of life changes and lore.