Deron Williams was, by far, the biggest trade of this year. There's really no doubt about it. He makes every other person on the roster better, whereas Carmelo, while a superstar, doesn't contribute in all of the ways Deron Williams does.
Williams is going to be the face of that franchise for many years and I honestly do expect Howard to leave the Magic at the end of 2012 for the Nets. Who wouldn't want to go play with Deron Williams? This will only happen if he is willing to give up being the "man" on his team, and I'm not sure if he'd be willing to do that. I think that would be the only thing standing in the way of joining the Nets honestly.
Anyways, I digress. Point is, getting the best point guard in the NBA was a better trade by far.