While much of this list will mirror Ghostmind's, here are my top 5 sets released in 2012:
(If I got the B Wing or Bag End those would probably get in here. 2013 for me I guess)
1) 10228 Haunted House
Great construction (ignoring the issue of bag #1, which could have been easily fixed), but amazing on display. A large scale dollhouse approach to a building that makes Town Hall look pedestrian for the best building of the year. Different from the modular series due to its dollhouse style hinge construction, and for building detail on every single side of the building. It looks magnificent once done, and comes with a great collection of mini figures.
2) 10225 R2-D2
Who doesn't love the lil Astromech droid? A very neat and fun build, and unlike any other I did over the year. The entire thing comes together very nicely and makes a wonderful display piece while also retaining some playability. The head is a turn off to many, but I am not sure how else it would have been achieved while keeping the same scale.
3) 6869 Quinjet Aerial Battle
Probably the biggest surprise of the year for me. The other Avengers and Marvel sets were decent, if uninspired. I had gotten them solely for the mini figures, so my expectations of Quinjet were not that high except for a parts and figure pack. But lo and behold, it is one of the best ships out there. A very solid and hefty ship once completed, and surprisingly complex in the assembly. There is great SNOT work everywhere on it, and lots of playability. And this set is the king for "Swooshability" right now. You just want to fly it around and make it blow stuff up.
4) 9493 X-Wing Starfighter
One of the first sets I bought in 2012, and what reawakened the beast for LEGO. This set sent me on the path to the Dark Side. Nice scale, and a very neat build. Many rightfully refer to "dat nose" while constructing it. It is quite cool how that part looks so simple but comes together so uniquely while building it. It looks great on display, and like any good starship, it is also highly swooshable. Bonus: it comes with a Porkins mini figure.
5) 9469 Gandalf Arrives
There are larger and more complex Lord of the Rings sets out there. Almost all of them are great sets and much better than initial pictures made them out to be. But this set is absolutely perfect at using a bare minimum of pieces and complexity while simultaneously capturing the mood and look of the film. A very quick and neat build, and also comes with one of the brand new horse figures.
Honorable Mentions
9495 Gold Leader's Y-Wing
Nice size, and neat construction. The use of smaller parts to achieve implied detail is exceptional, and it (along with the TIE Fighter) make a perfect, and almost required, complement to the X-Wing.
9465 The Zombies
Exclusive to Target and LEGO stores, and now sadly gone from all the shelves. This Monster Fighters set not only came with great Zombie figures, but also featured the best vehicle from the entire Monster Fighters line. The fact that it has interactive graves is just bonus.
3061 Friends' City Park Cafe
My dark horse entry for the year. A small, fun set. You get a lot of neat and different colors in the set, but the little details as you go are much like the modular series. From tables and benches, to ice cream makers, and even a cheeseburger, this set has so many neat little flourishes. Do not be turned off by the Friends line. There is actually a lot of neat stuff to be had from it.
Mini Figs of the Year
Available only as a freebie from LEGO back in the summer, this figure is both comical and fun to fit in with any theme. I know I have gotten a lot of mileage out of it, as Hulk goes with everything. Yes, I would actually put this figure above Deadpool.
King Theoden came with the Helm's Deep set. The intricate paint job on the armor and helmet are what set this figure apart from all others in the LOTR line.
It's a guy. In a bunny suit. 'Nuff said.