What was the best MMO expansion of all time?

Whether or not the expansion model is here to stay or on the way out in the industry is a topic for another time. Instead, I want to talk about the best expansion of all time. AOL's Neverwinter Nights supposedly had an expansion in 1992, but the modern age of expansions didn't get going until 1998's Ultima Online: The Second Age. Since then we've seen scores of add-ons to our favorite MMOs. In 2012, we had nine major expansions enter the field (and we're not even counting content updates).

A great expansion significantly improves upon the game that came before. Perhaps it adds a wealth of new content, is just superbly done, or introduces a beloved feature to the game. For you, it could be the expansion in which you fell in love with a certain MMO.

Don't hold back here; if there ever was a time to step up and be bold, now is it. Let's fight it out in the comments over the best expansion that was, is, and ever will be!

TH