Well, Jeff the obvious, he's clearly very talented and technically fantastic, but I totally understand not being excited by him, he seems to process the game incredibly slowly, I pretty much wrote him off last spring when I watched him against Anderlecht but the move to CM and the inevitable Diaby comparisons mean I'll hold out hope for him, sometimes also with these graceful, lanky Pogba/Diaby types they actually look like they are moving/processing the game slower than they actually are, because of their very nature, so I rather optimistically, I admit, hold out hope for him.
Maitland-Niles because he seems to be the one most on the fast-track for Arsène, and although I've never really seen anything that sticks out to me from him, there doesn't seem to be much reason why he can't be a good defensive midfielder, he's got more quality on the ball than Coquelin or Elneny, certainly offers himself more than the former, and seems to have the necessary athleticism and positional sense for the role. Wouldn't say he excites me so much as I'd say he's the one from the current class most likely to make it as a squad player.
Willock is the one who definitely excites me the most of the three, his close control and dribbling is immense, and he seems to have a pretty good brain too. But he may have too much of a passive personality/Arsène always holds back the skinny/under-developed types like him (which is nonsense, IMO, but I digress). Also maybe the contract issue in there but that's pure speculation.