John Simian played the severely deranged Mattress during the Tenth and Twelfth Doctor's era.
Well, he's supposed to be the Master, but he often acts more like an evil hybrid of the GOD-TIER Doctor, the Underrated Doctor, the Based Doctor, the GOAT Doctor, and the Overrated Doctor. Not a bad villain, actually pretty good, just...not the Master.
If John Simm and David Tennant switched roles, would it have improved the Tenth Doctor and the RTD Master? Yeah, probably.
Simm /who/ Edit
Simm has a loyal following on /who/ known as Simmchads.
Simm appears in Utopia, The Sound of Drums, Last of the Time Lords and the two-part Christmas special The End of Time.
And now Series 10. Officially confirmed now, apologize to the Moff.
And now tired old memes
that only the oldest, saddest virgins on /who/ would have any recollection of.
If I see that fucking Master one more time...
Seems that his character may finally be treated with some respect.