Patton in goal for me, but you could easily argue for Beggan too.
McBrearty nailed on in the forwards. McFadden is important for Donegal, but I don't know that he would merit a place in an all-Ulster midfield. But that may speak more to the lack of outstanding midfielders.
Think Morgan is the best keeper, Beggan and Patton are probably better on their kickouts but Morgan isn't far behind and is much better at commanding his area, a better shot stopper and much more comfortable with the ball in hand than both Patton and Beggan. I don't think Beggan is that great of a goalkeeper once you dismiss his kicking ability, he's been fairly poor for a lot of goals in recent years - the goal Fermanagh got against them a couple of years being the most obvious one, for a big guy he doesn't take charge of the aerial balls.
McBrearty hasn't looked the same since coming back from the ACL, maybe this year he'll return to his previous form but I thought he was poor last year.
As for midfield, I think McFadden is a very underrated player with Cavanagh in decline and the likes of Kearns, Burns, McGee and Kennedy etc still young and needing to show their worth over a few seasons. Niall Grimley is another very good player but I think McFadden has the better defensive nous there. McKiernan is an absolute certainty for me, a rolls royce player who walk onto any county team in the country, including Dublin.