I would clear naxxaramas and stay in kt's room till the end. Why? to this day nothing has in my opinion come to touch the quality that 40man naxxaramas was and the type of effort and coordination it took to clear it (40man c'tun during vanilla might be equal). Wow ended for me the day the pre-patch for burning crusade came out.
the elemental invasion, the dark gate opening events both pale in comparison to the original scourge invasion as far as world events go. even deathwing's event was worse than a dissapointment.
Those of you whom have not truly experienced those things i mentioned above, well i feel sorry for you. These are the memories i will take with me when wow finally gets shut down. it will live in my memories alongside all the wonderfull pre-cu star wars galaxies memories i have. unlocking my jedi during permadeath timeframe. putting on my first full set of composite armor .. later bh armor .. well you get the idea.
The only thing then i can hope for is that the next mmo i try will fill keep me as entertained as those two games did, and give me new experiences that i can keep in my mind and remeber alongside my other great mmo experiences.