My #1 pick is the Freightliner Cascadia.
I've (somewhat) patiently waited for the introduction of the beloved Cascadia since the initial release of ATS and the amount of detail SCS transferred into the game is amazing, just as the other models.
With the right engine/transmission combo, you get an all-round hauling machine. Although a lot of heavy haul drivers would prefer Freightliner's sister brand Western Star and their new 49x model, it's basically a "Cascadia 3500HD" if you will - just a more rugged look and some small tweaks under the hood when it comes to in-game performance. I personally drive a manual EF 18 speed with the Cummins X15.
The interior is the best I've seen, though that of the International lonestar and recently added LT is a close second. If there were anything negative about the Cascadia's interior that I'd mention, it would be that there is no [Fuel Mileage Untill Empty] info in the instrument cluster.
You see Cascadias all around, it's one of the most popular fleet trucks in America. They work hard and look great while doing it.
As a bonus this is my main truck, Crimson, that I've driven since Freightliner's addition to the game and I do plan to keep it and drive it for years to come.