Route or vehicle? I've only been on so many different routes and vehicles?
As for routes, I am torn between the Northeast Corridor and the London to Paris lines. I've done both and am fond of both routes. Taking the Eurostar however was a bucket item list for me, so I guess that slightly edges out having taken Acela?
As for vehicle, I don't really have any. I am not super big on transit vehicles in themselves and while I know a lot about the trains here, it's circumstantial. I couldn't tell the difference between a train from Siemens and one from Alstom for example.