Since you said used I will recommend a Suburban (or Yukon XL). For that weight a common 1500 series will probably work. But, if you can find a 2500 series it would be better. I bought a 2009 2500 Suburban to pull my 6,500 pound boat and it does a nice job, even with up to 9 people inside the vehicle. If you get a 2009 or newer the six speed automatic will automatically downshift when braking or to hold he cruise control speed. Earlier models didn't do that.
They stopped making these in 2013 so that's the newest you will find.