How many major warships (cruiser size and above) were ever hit by a torpedo from an MTB, much less sunk?
Italians MTBs bagged two Austrian capital ships during WW1... SMS
Wien in harbour and SMS
Szent István at sea.
British MBTs operated against Soviet vessels around Kronstadt during the intervention in the Russian Civil War. They managed to sink the cruiser
Oleg and heavily damage the per-Dreadnought
Andrei Pervozvanny. Some sources claim they also damaged the Battleship
Petropavlovsk, but other sources dispute this.
HMS
Manchester was scuttled after heavy damage from Italian MTBs during Operation Pedestal. Not strictly a torpedo boat attack, but HMS
York was badly damaged by Italian explosive motorboats in March '41 and ultimately scuttled before the fall of Crete.
HMS
Newcastle was damaged by Schnellboots in June '42. Quickly skimming wikipedia there are claims that S-Boots claimed a second cruiser damaged, but the name of the vessel goes unmentioned (and I can't be stuffed trawling through the history of every cruiser in service in the Atlantic and Med during WW2).