My opinion:
The GM 6.0L is the most proven, but leaves a lot to be desired in regard to towing.
The new GM 6.6L is a beast, but has only been out a couple of years. It’s paired to a six speed transmission.
The Ram 6.4L is a good puller, but anything with cylinder deactivation starts out with a giant reliability weakness. The first version came out in MY 2014 and required 89 octane fuel. It is similar to, but very different than the 6.4L used in the Dodge sports cars. I believe it was in MY 2019, Ram moved from a six speed to eight speed transmission.
The Ford 7.3L is the current big dog, but has only been out around as long as the new GM 6.6L. In regard to purely towing capability (legal), this is the best.
The Ford 6.2L has been around, in the “Super Duty” line, since MY 2011. For MY 2017, the engine was revised and produced more torque at a lower RPM. This engine has a great reputation, for reliability. Prior to being used (in a de-tuned setup) in the SD line, it was used in the MY 2010 Harley Davidson F-150 and SVT RAPTOR. From MY 2011-2014, you could get the (full power) 6.2L in a regular F-150, as well.
The GM 6.6L, Ram 6.4L, and Ford 7.3L are used in the larger chassis cabs. The 6.2L never was used above an F-350. Prior to the 7.3L, the 6.8L V10 was used, in the F-450/F-550’s. The 6.8L V10 was put in F-250 to F-350 trucks, prior to MY 2011.
The 6.2L is a SOHC type. The rest listed are the old school pushrod.
With any of these, getting the biggest numerical value rear end ratio is a good idea, if you’re towing. My Ford 6.2L has the 4.30. None of them will get good fuel mileage, towing heavy. Gasoline engines make their power higher in the revolution range, these are large engines. Put that together and you get poor overall fuel economy.
My 2014 is a crew cab chassis cab with a ten feet long flatbed. It weighs around 9k pounds, with me driving and no load. My dump trailer is just over 5k, empty. It’s rated to around 15k and I’m frequently around that weight. I’ve pulled a few trailers over 20k and it does fine. I wouldn’t recommend the 6.2L, for that weight, everyday, but the Ford 7.3L is rated for it. One thing I like about the F-350, with the 6.2L, is it uses the same transmission as the much more powerful turbo diesel (a couple of small parts are different). I believe you will find this is true of the other brands, but I’m not sure. Note, the F-250 uses a “weaker” transmission, but it’s still highly regarded.
ETA: I forgot to mention the Ford offers a 10-speed transmission, now. The six speed is still available, but limited.
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