What is the most modern ship HMS Dreadnought would have a reasonable chance of beating?
Short answerA River-class offshore patrol vessel is one example. Dreadnought could probably defeat many modern ships placed within her gun range, but weapons able to attack beyond line of sight change the comparison.
NotesA River-class offshore patrol vessel is one modern opponent HMS Dreadnought could defeat. Ships able to attack beyond line of sight pose a different problem for the old battleship. If the opponent starts within Dreadnought's effective gun range, however, her heavy armour-piercing shells would make her dangerous to many modern ships. Eight 12-inch guns could fire in the salvo being considered. A modern 5-inch gun could cause damage, but the assessment favors Dreadnought once her main battery can bring effective fire onto its target.