Gun to my head I think Bruno in all honesty, others are younger with higher ceilings for me.
Then you have the issue of replacing them for Bruno you have Tonali, Willock, Joelinton, Anderson, Longstaff, Miley is cover enough for CM. Out of our wingers though Gordon is way out in front of Barnes, Murphy, Almiron and Isak is clear of Wilson with one leg so would be a real problem to lose either of them whereas Bruno has cover already.
As GG say he is also the most profitable one so if you can get the £100m reinvested well it could go a long way towards replacing a lot of the dead wood with a few smart buys - maybe even another £35-40m CM who's as good as him.
My only hope would be that he goes back to France with PSG or to Madrid (the only clubs who can really afford him).