The "cheapest public transport" thread made me think about the other extreme, and that made me search for this one. Before I opened it, my two guesses were the Vaporetto and the HEX.

I'd probably give Venice the prize for "most expensive" since there really are no other less-expensive feasible alternatives for getting into and out of your hotel with luggage. London does have cheaper alternatives...plus multiday unlimited Tube passes and Oyster rates aren't that bad. (Vaporetto passes are expensive no matter what...multiday rates don't drop off dramatically to become cheap per-day.)

I've taken some expensive transit options to/from far-flung airports (NRT comes to mind), but this thread seems to be more about purely intra-city transit.