Ford Super Duty diesel engine?

Which is the best engine for diesel engine for a ford truck. People are telling me the best one is the 7.3 because it has the less problem but someone else to me is the 6.0. Please give me an idea on which is the best selection because I'm ready to purchase a used truck in a couple of months. I know these a 6.0, 6.2, 6.7 and a 7.3. I know they stop make the 7.3 in 2003, I can still find them. Thank you guys for your help.

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