my cousin gets about 14-15 highway in his duramax, he hasn'thad it long, about 3 years, and only puts about 2-8 thousand miles a year on it towing his racecars.
Personally from growing up in Redneck Shanggrala, where it seems every body has a diesel pickup around Danville. I like the cummins > powerstroke > duramax
But to defend any of these, realize you are going to develop problems if you drive it back and forth to the grocery store (unless it's 30mi. away) Diesels are happy towing and running constantly, any of the big three diesels will last 200,000 miles easily with proper maintanance and proper warming cycles. I was taught that if you aren't going to run a diesel for a half hour at least, it's just better not to start it in the first place.