For the first "Mister Terrific" you ever read about, here are the possibilities to consider:
* Golden Age Mr. Terrific (Terry Sloane) - 1st appeared in Sensation Comics #1 (January 1942) / death of character in Justice League of America #171 (October 1979)
* Michael Holt - 1st appeared in Spectre #54 (June 1997)
NOTE: not differentiating between pre-Flashpoint and New52 versions
* Kingdom Come / Earth-22 version of Mr. Terrific - 1st appeared in Kingdom Come #1 (May, 1996)
* Liberty Files version of Terry Sloane - 1st appearance in JSA: The Liberty Files #2 (March 2000)

NOTE: Not including Earth 2's "Terry Sloan" (no "e" on end) of the New52 since he went by the name "Mr. 8" in that version.