All things being approximate, and thus a standard atmosphere assumed, the answer to the first part is sea level
Mach 1, speed of sound, is solely dependent on temperature. Highest TAS is at highest temperature for a given mach, regardless of altitude.
Death valley, perhaps, in the real world.
Second part, highest tas for a given IAS (I'll assume no position error, so ias = cas). If we're talking a vessel capable of deriving lift from the atmosphere, then the space shuttle during reentry would represent this altitude. Very low cas, very high tas, at point where it begins to fly again.
Short answer, as high as possible.
Hawk