4. Josh Brolin
Given the nature of Brolin's appearance in the third film, it is highly unlikely that he will be asked to reprise the role of young Agent K again, lest the franchise be criticised for inventing ways to write him in which don't necessarily fit with the new storyline. The risk here would also be that MIB becomes as much about time travel as it does about protecting the Earth in the present from inter-galactic threats. Still, if it were possible to write him in plausibly, it would no doubt be a popular move with fans who enjoyed his performance in MIB 3. Given the amount of reboots and re-castings in Hollywood history, most recently with J. J Abrams' Star Trek, taking on a role that another actor has effectively made their own is always going to be risky for any performer, as there will always be a natural tendency to compare and contrast between the two. Brolin swept that risk aside and delivered a performance you could easily have imagined from a young Tommy Lee. Every inflection, each growling stare, even the timing of his dialogue was in keeping with the best aspects of Jones' previous offerings, but with a younger, dare I say 'happier' twist that you'd naturally expect from K's younger and less experienced self. Probably the crowning jewel of MIB 3, and a welcome addition to any future MIB film, however unlikely it may be.