Like he said. Make an integer variable somewhere that remembers the highest score. (Make a save game file that contains it, if need to be remembered every time you play. or a game instance if you reload the level, and want to remember this between loads).
Compare the current score with the high score, and if its bigger, set this as the new score.
Then create a widget that looks this score up, and set this as a text in the widget.
So the difficulty also depends on how, when, and where you want it to show 