Personally I think Lampard is most likely to go. He's only got 2 or 3 more years left at the top and it's a chance for him to join Mourinho once again.
You could say the same for Gerrard but IMO Mourinho would want Lampard more than Gerrard and I really doubt Gerrard's family will want to move to Madrid.
Ashley Cole could be quite likely to go as well, there's not much left for him in England with his split from Cheryl, I think he'd want to get away and work with Mourinho again.