No question the 7.3L is the most reliable of the bunch, but don't believe all the hype you hear about the 6.0L.
The 6.0L, or V365 in an International, is a superior engine to the 7.3L. What killed its reputation was a rush to production, combined with the parts Ford used to shoe-horn it into a pickup truck. Plus the fact that Ford used patented filters, meaning no one in the aftermarket could copy it. Not knowing this, lots of people bought sub-standard filters (oil and fuel), rather than purchased over priced filters from the Ford dealer. It was so bad, Ford made a video for it:
Also, there is a problem with people mixing different coolant types, causing the oil cooler to clog. Plus the EGR cooler is notorious for failing, along with the EGR valve. None of these items were found on any 6.0L engine found in an International truck, because they are heavier vehicles with different (looser) emissions requirements. As a result, the reputation of the 6.0L (VT365) in International trucks is rock solid.
If I were in the market for a diesel, I'd look at 6.0L real hard. Many people avoid them, making their prices relatively attractive compared to a 7.3L. But before I bought one, I'd run full diagnostics on it (Ford dealer should charge $100 for this) and I'd take engine oil and transmission fluid samples and send them to Blackstone Labs for analysis ($50 plus postage).
But back to your original question, 2003 was the last year for 7.3L engines, the year was split. Some 2003's have 7.3L's, but most will have a 6.0L. The last year for the 6.0L was 2007. The exact date range for the latest and greatest 7.3L was 99.5 to 2003.25. If you find a pre-99.5, Riff Raff Diesel sells a kit to bring it up to post-99.5 status