I've got an 07 classic Chevy 2500HD. I would really say you can't go wrong with one! The only issues I have had thus far are:
1- a bad stereo speaker
2- a reset of the computer due to the original programming having too tight of parameters and thus throwing codes when nothing was wrong
3- a bad EGR valve solenoid
They were all covered under warranty.
I have 95,000 miles on the truck thus far.
I am not the nicest of owners, it's the most powerful vehicle I have ever owned so, I at wide open throttle most of the time!
I haven't towed anything super heavy with it yet, heaviest was a Dodge Ram Cummins dually on my car trailer that I towed from Alabama back to NC for a friend (probably 8000lb total trailer weight?). Usually tow between 5000-7500lbs if towing anything but that's only about once a month or so. Good acceleration even with a heavy load! The manual feature on the 6 speed Allison is fantastic when towing, can engine brake nicely!
I still kinda wish it had a solid axle frontend but, to be honest it doesn't need one. If you keep it pretty stock (the only mod I have done to it is 265/75 R16 tires since the stock 245's were WAY too small looking) and drive it appropriately (it's not a rock buggy!) it lasts a good long time and gives a nice ride too!
Mileage is good for a heavy truck that is driven by an owner with a lead foot- I typically see 15-16 mpg. I have heard of much higher but, the highest I have ever hit was 17.
My vote is for the 2500HD! My second place would be a tie- either a superduty crew cab with a Cummins swap or a mega-cab Cummins (depending on what bed length ya need).
My only regrets with my choice are- I wish I woulda bought a crew cab (the extended cab can be tight for 6 people) and I wish I woulda bought a GMC (to me they look a bit more upscale and it would match my Jimmy better (would have both GMC!)).
Overall though, I love my 2500HD!
Here's a little pic whoring, a shot of my 2500HD towing my Jimmy:
I do have to say though, nowadays it's kinda hard to go wrong, Chevy, Dodge, Ford, and GMC all make great trucks! As noted before, they all have their weaknesses but, they all have great strengths too! Good luck with your shopping bro!