Okay I will bite. part of the reason people dont answer is because if you search about pros and cons of each motor will will find tons of threads regarding this question.
However I will help you out. I have owned 4 diesels now. All fords. First was a 7.3, second a 6.0 which I sold to buy a 6.4 buy got screwed on a deal and ended up back in another 7.3 Now own a 6.4. If you are looking for something to keep STOCK buy a 7.3. They have the least amount of emmisions equipment and run for many more miles because of that.
Here are some key notes
The 7.3 SD years(1999-03) dont come with as many creature comforts as newer vehicles (which can be added if you choose....newer seats, newer suspension, audio, front end etc). They generally have higher miles due to their age, which with the 7.3 isnt really a big deal. As many have been known to run well over 500K miles.
The 6.0 is a great motor but as with any vehicle you buy has its flaws. The early 6.0 had injector problems. Some trucks need studded stock and some run many miles tuned without ever needing them (ie: my 6.0 which was an 05. I ran innovative diesel tuning on the truck for the 30K miles i owned it, and never had any puking. And didnt drive it easy by any means) If your truck does need studded it can get quite costly as it is a cab off procedure. You also want to delete the egr or put a bullet proof one on if you have to deal with emissions. I did this along with a new oil cooler and always watched my ECT/EOT temps. I also dealt with stiction issues with my injectors. However this was easliy fixed by running rotella t5 instead of standard rotella.
The 6.4. Too has it problems. Radiator issues, like the 6.0 they had head bolts that stretch and fail needing a cab off to replace and install head studs. Then there is the dreadded DPF. This along with the EGR system is said to greatly reduce your engine life. The dpf also allows the engine to go into a regen mode to burn off the extra soot that it traps. The engine uses fuel to do this which in turn leads to worse MPG. You however can run tuning which allows for the DPF to be deleted and the egr turned off. This is costly. A good tuner such as H&S and spartan will run you close to $1200.
the 6.7 I cannot report much on as i havent owned one and havent read into them much. From some of the threads I have clicked on I have seen alot of burned up stock turbos for whatever reason. But I will leave that up to you to research now that ive gotten you started.
IMO the 6.4 if you are okay with mods is the best truck ive owned. All of my trucks now have had tuners, intakes, exhaust etc. And my 6.4 although it currently sits at stock height and is a 6 speed manual where my others were autos. It gets the best MPG. Im currently deleted/tuned and yield 18-21MPG depending on my driving. Obviously as it is a newer truck the creature comforts with this truck are amazing but my last 7.3 I swapped in 07 front rear bench seats, and indash with navigation, newer front end, 08 tailgate and was still in to it WAY less than the cost of my 08.
hope this helps and good luck on your purchase.