Can-O-worms! Its like asking who makes the best bow for elk or what is the best caliber and bullet for mule deer. ;D
I own a 2004 LB7 Duramax...It’s been a great truck and gets 21 miles per gallon. I have 155,000 miles on it and it still has the factory brakes. I have a 30' Fleetwood Wilderness and the truck tows it very well. Replaced 2 front hubs, a steering shaft sheath as well as put in new injectors. It was all recall stuff and covered by GM. My buddy has a 2005 and his is trouble free and my other buddy has a 2007, no trouble either. Bitchin' trucks
I have a buddy who has owned 3 Cummins' and he loves them and another buddy with a Powerstroke and he loves it too. Basically its hard to go wrong with all three and probably comes down to personal taste regarding appointments - comfort etc. I will say though that out of the three brands noted, my friend who owns the Ford has spent the most in maintenance.
My two cents for what its worth,