Viktor Belenko, the pilot that defected in Japan, reported that the MiG-25 was theoretically capable to reach over Mach 3, but there were heating, controllability and engine limitations. Max operative speed was 2.83 Mach at high altitude.
I found http://www.wvi.com/~sr71webmaster/mig25.html and http://en.wikipedia.org/wiki/Viktor_Belenko
And, for this, I suppose the MiG-25 is the fastest turbojet engine aircraft (the Blackbird is a sort of mix between turbojet and a ramjet).