With so many map expansions, it might be difficult to figure out what one to buy first. This is mostly an opinion based one. There are a lot of good maps and they all come with their own specific rules. So something that you like might not be something that someone else likes.
In my eyes, if you are looking for some of the best maps check out the United Kingdom expansion and the Japan expansion. Those maps have the best ratings among the Ticket to Ride community. Still, make sure to read up on some reviews to make sure that the mechanics are something that you would enjoy.