I bought the LevelD 767
http://www.leveldsim.com/, and it is spot on in many cases. There are a few really advanced things not modeled, but everything the pilot normally interacts with is there.
The PMDG 747 is supposed to be superb as well
http://www.precisionmanuals.com/priv/pages/prod/fsx/744x.html. I don't personally own it, but it seems to have a very loyal following.
I had downloaded the freeware iFly 747
http://www.simviation.com/fsxjets19.html, though haven't gotten the chance to really play with it. I did take a look at the FMC and was very impressed to be able to access the CMC (Central Maintenance Computer) pages and some of the advanced engine snapshot functionality. I don't think it's totally functional, but the pages are there. With that level of detail implemented, I'd assume the rest of the aircraft is pretty accurate.