While Watch Dogs has a higher attach rate on PS4 in NA (which is impressive) and the fact that it's a new IP for Ubisoft, I'm honestly more impressed by MK8;
Firstly, we all know that sandbox style IPs ALWAYS do well and Watch Dogs looks like a tech inspired GTA so it was bound for success, not to mention its massive marketing campaign.
Second, and the most important reason in my opinion, MK8 might have just brought back the Wii U from the dead which EVERYONE thought was impossible. I personally expected it to move a few units but not 4x the amount, given the target demographic and Nintendos relevance in todays market space. We'll know within a months time how well the platform will be trending after the boost but the sheer numbers put up in TWO DAYS on a console with a very limited install base is damn impressive for a title that's already had 7 previous releases.