I would recommend getting a Mesa, easy to get in Online if you know what your doing.
I also recommend the Dubsta because it is AWD. Which I have in my garage.
I own a Rebel 4x4 going for the baja look. I also know where the Rebel 4x4 clean version spawns. But you can't mod it like the rusty/dirty version.
I own Trevors truck and it's pretty good for off roading or going over mountains but isn't the fastest, but it's very durable can take a good beating.
I also have a Sandking SWB, great for off roading also, because it sits so high it can go over go high bumps on hills that most other off roads struggle with.
The Merryweather Canis Mesa is the bomb, it's perfect for off roading and has a good lift kit on it. It's built like a beast/tank and can take a good beating, also with turbo it's a mountain eater.
I would recommend a Sandking XL/SWB, A Rebel 4x4, or a Merryweather Canis Mesa.