for me the 5d mIII
Adobe has online videos on how to use new features in both LR4 and LR5. For LR4, the update to the 2012 process is very important as well as soft proofing, for LR5, the Upright feature and the Radial filter are notable new features. Photobooks might also be something to consider.
You should definitely go to Adobe TV and view the julieann kost videos, some subtle things might be easy to miss, but be important to you.
Here is a 40,219 x 11,522 shot taken with my motorized head:
Great Falls by scrappydoggy, on Flickr
The moving water was a nightmare from a blending standpoint. The photomerge in CS5 was otherwise easy, albeit time consuming.
I have a small concern about the leather/soft/rubberized parts of my 50D. As you know its normal color is black, but mine is showing a peculiar whiteness within this black color - specially in the "channels" or "grooves" of those rubberized parts. No problem with the metal parts though. Any idea about this? Thanks.