It depends what your interests are. Canadian Mountain Passes and West Highland South are the two earliest routes. They are both very long. Detailing and landscape modelling has improved with their more recent releases, but I still enjoy both of them, with the West Highland being my favourite, in part because I'm from the UK and I have travelled that line shortly after the time period it's set in. Of the newer routes I've got, my favourite is Arizona Divide which is a mostly fast freight route and feels much like the real route when I travelled it in 1988. I would say that Milepost Simulations produces good, but not outstanding DLC, but they are decent value for money. At their current sale prices, which are the lowest they've been, I don't think you'll be disappointed.
Get something if you think you will play it - don't get it because it is cheap Try Googling the route name and you should find a video of the route and see what you think.
