Regarding British generaliship during the unfortunate episode in the history of the Board of Trade, I have little to add to Jonah’s correspondents. Cornwallis was the best of the lot; he had the misfortune to be pitted against Nathanael Greene, the best of ours. Note that after Yorktown he went on to success in India, beating Tipoo Sultan.
Probably the best British armies abroad were Marlborough’s (unless one goes back to Crecy and Agincourt, though then “France” did not count as abroad, at least as far as the kings of England were concerned).
Wellington of course is close second, if not tied for first. Note though that his Peninsular veterans did rather badly in the War of 1812 (Old Hickory said we cud take ‘em by surprise . . . ).
Montgomery’s reputation is still a battlefield, and I don’t know enough to visit it.