I'm surprised no one else has mentioned metro/light rail stock yet, as I didn't discount it.
Possibly because you... er, generally... don't spend as much time on them, so you tend to view one as a moving pavement. I wouldn't want to do Canary Wharf to Reading in a DLR unit, but to Bank was ok ( and given the view out of the old units, sometimes kinda fun ).
Although now of course you're more likely to have a spartan bus of some description imposed upon you, with the relative scarcity of accessible high spec coaches![]()
Buses are getting a bit scarce at times too...