dollar for dollar I would recommend a LBZ (06 to early 07.5's) due to power, lack of emissions crap and how much power you can make out of em.
If your wallet calls for a LB7 / LLY trucks (01-05) thats ok too, all great motors, great fuel mileage and awesome dependability.
I sell cars, the market sucks. Find a LBZ Duramax you can STEAL and you'll never look back.
2005 GMC Duramax CCSB SLT. White / gray, roof, leather. All stock for now.
*SOLD*2004 GMC Sierra 2500 SLE. Regular Cab. Black / Gray, LLY Duramax, EFI LIVE, Kennedy 4 into 5" exhaust, 6k HID's. 20+ MPG tire smoking monster!
*SOLD* 2008 GMC Sierra 3500 SLT - Crew Cab Long Bed SRW. 8" Fabtec Lift, 24" wheels, 37x13.5x24" Toyo open countries, Dynoflow exhaust, Air raid CAI, Bullydog PMT.
*SOLD* 2006 GMC Sierra SLT. 4x4 Crew Shorty LBZ DMAX. Burgundy / Tan Leather
*SOLD* 2004 Chevy Silverado LT. 4x4 Crew Shorty LLY DMAZ. Charcole / Gray Leather