You can't go wrong with any year chevy, as long as it was well maintained.
GM is the only manufacturer IMO that doesn't have "certain years to stay away from" with the exception of the early Diesels. If your a getting a newer one 99 up they are AWSOME! especially the 6.0l and the Duramax. The old motors were great too, it's just that the new engines are some thing fresh and new with alot more power. also with the big block and the Duramax you can get the Allison automatic which is a big step above just your average heavy duty pickup trans. The allison is a scaled down meduim duty transmission that is gonna take more abuse and heavy loads than the 4L80E - still a great trans.
The 3500hd is the exact truck I want, it has the 15,000gvw, but they didn't make them with 4wd.
Ford is good too, but you gotta do your homework before you buy one of these as different years have different problems- New 6.0 diesel has problems, the torqshift trans has problems, the mid 90 transmissions would blow out if you back up under a heavy load. The v-10 blows spark plugs out of there threads.
The 7.3 PSD was the best motor ford had in it's trucks.