Bryan Cranston, Aaron Paul & Dean Norris
Yes, all of your favourite Breaking Bad stars have made an appearance on The X-Files at some point in time. Which shouldn't surprise anyone, given Breaking Bad's creator Vince Gilligan wrote 29 episodes of The X-Files and produced many more.
Bryan Cranston starred in a Season 6 episode called 'Drive'. He played a violently ill man suffering from a contagion who is forced to keep driving west in order to relieve his pain. It was one of the most positively reviewed X-Files episodes of all time and Cranston's stellar performance is what got him the role of 'Walter White' on Breaking Bad.
Aaron Paul starred in a late Season 9 episode called 'Lord of the Flies'. He played the leader of a group of teenagers who were filming a Jackass-style TV show when a stunt goes wrong, killing one of the group. Yes, that's Jane Lynch in the background, of Glee fame.
Dean Norris starred in an early Season 2 episode called 'F. Emasculata'. He played a U.S. Marshall tasked with hunting down a couple of escaped fugitives who are infected with a virus.