Everyone will have their favourites, but I'd say:
- all the Swiss options, with RhB at the top, plus all the long tunnels old and new
- trains on boats (to Sicily and the Fehmarn ferry (sadly no more)
- Przemysl-Lviv-Kyiv (Ukraine)
- coastal routes (Genoa-Nice, Naples-Sicily, Far North line, Cambrian coast)
- river gorges (Lune, Elbe, Rhine, Douro, Inn, Sava)
- bridges (Conwy, Forth, Tamar, Oresund, Tagus, Friendship (RO/BG), Hohenzollern (Köln)
- scenic heritage lines (notably the two D&RG remnants, Colorado/NM)
- any route with DB's ICE-T
- any fast line into a London terminus
- any Pendolino ride, but particularly Lancaster-Glasgow