There is no "best", it always depends on the preferences of the player. Buy what you like the most and make it your "personal best".
Are you talking about the starter packs or is it about individual dlc? If you're unsure, you should go on YouTube and look up the route, check forums, check reviews and see if you'll spend enough time on it for the purchase to be justifiable For starter packs, they are all pretty good. Depends on what you like. Us stater pack has peninsula corridor with the mp36 and mp15 which are really good. It is a diesel route, nice trains, lots of service variety, challenging brakes on the f40, etc. Freight is present and each service is unique, same with the shunter. It is getting a preservation update too so it'll be even better. German starter pack has a German dlc (rsn) and the 363 and 155. It's mostly a freight route with a few passenger runs. 363 does shunting. 155 either layers or subs into already existing freight services. The route itself comes with the 143 and the 185.5. It is a scenic route with a bit of curves. 363 and 155 takes some practice to drive. Uk starter pack is lovely too if you like br blue diesels. You get tvl which comes with the 101 (manual gear and manual lapped brakes, so a bit challenging), 37 and 08 which are really well done locomotives. You get class 20 and 31 as dlc which add layers or sub in. If you're looking at individual dlc, most are pretty good. German routes tend to be scenic and challenging in their own way. If you're on next gen (xbox series), other German routes layer/sub on each other so your route gets busier and more varied. I would recommend each German route except for rapid transit (not the best). Uk routes are also really good. I like all of them. Most are modern electrostar routes if you like those. There's also br blue routes if you like old, thrashing diesels. Gwe is a all diesel route (not br blue). Us routes are quite decent. You've got 4 passenger routes to choose from (lirr, peninsula corridor through the starter edition, necbp, harlem). lirr is very empty and the sounds aren't that great so I wouldn't really recommend that. And some freight if you're into that (really challenging tho). Then you got other countries such as Switzerland and France.
