What 4X4 would you recommend for towing a horse trailer?

I am looking for a 4X4 to tow a double horse trailer with two 15.2 hh horses. The vechicle needs to be powerful enough to tow up steep hills (live in wales!) and i know certain makes are awful for towing. Any ideas or advice apreciatedWhat 4X4 would you recommend for towing a horse trailer?
Obviously the answers you have received so far are from Americans who didn't notice that you live in WALES, in the United Kingdom.
I have a 2003 Chevy 2500 HD diesel 4x4 4 door. I also have a Predator programmer for added horsepower. The Chevy has a Allison transmission, nothing beats it.What 4X4 would you recommend for towing a horse trailer?
I'd get a diesel. Ford, Chevy or Dodge 2500's.
Chevy, Ford, or Dodge diesel truck. They all tow nice. I am a Chevy fan, but think the Dodge has a little more power. I have a 2006 Dodge (traded in 2004 Chevy diesel) that I use to tow an aluminum stock trailer that holds up to 6 horses. It tows great!What 4X4 would you recommend for towing a horse trailer?
Ford F-350 turbo-diesel.
I like the ford deisels myself wit the 7 1/2 liter motor. Preferably extended cab to hold all of your stuff if you are hauling horses. Ford F-250 or bigger and Dodge 2500 or bigger have solid axles in the front. This gives better handling and traction than independent front suspensions like the chevrolets have. You can find a 96 to 01 models pretty reasonable. If they have 150,000 to 200,000 miles, this is no biggy for a diesel. It will be just broke in.
I second the previous suggestion, you need at least a 3/4 to 1 ton turbo diesel like a Ford F350. They have 325 HP and are geared low enough to pull up most grades without any effort.