Khan for me, stylistically most difficult for him I think, offers something different to most if not all fighters he has faced, Brook would be a good fight but it makes no sense financially as Brook holds a very small name in the US still. GGG and Crawford just wont happen, Matthyse would be easy for him and Thurman is a good fight but it might be too early for him. I see a Mayweather win regardless of who he faces though.
Brook brings the IBF title, which will make the fight for the WBC/WBA/IBF/WBO titles which I prefer to see too. Mayweather knows that whoever he fights at this stage, people will tun in so he's going to aim for someone that can bring some more attention to add up to the insane attention he brings. Titles really mean nothing to Mayweather now and Brook only has one significant win worth bragging about. To be fair, Khan has had more big fights in Welterweight all alone than Brook has had his entire career. That's what matters and Khan easily wins this case.
Khan & Golovkin are the only exciting fights out there the most boring being a 41 year old Marquez rematch.