BML, SEHS, even ECML or BPO actually. All good routes though the last two are the chart toppers personally mate.
Another incredibly frequent route is the Bakerloo Line, literally being a rapid transit route and all.
I know it is technically possible, like always returning to timetable, or staying in free roam in timetable mode... but I would certainly welcome a mode where you could drive a chosen service back to back... like you would complete one way, and you could time-jump a bit before the start of return journey, change ends...
there are a bunch of lines where this is possible, and it makes for great shift kind of work. BKL is a great example of this, and a lot of branches as well, like the seaford branch and the st ives branch, which both have their own captive sets. EDN and cathcart circle is probably also good for this, since they are end to end runs. Often with these runs, there is little time gap between services so you can simply go back to free roam and walk to the other end of the unit.
If you haven't got London Commuter yet definitely get that one. Very busy route with a lot of services.