In the Marvel Universe, the name Mister Fantastic is most commonly linked to Reed Richards:
|Mister Fantastic (Reed Richards) - one of the founding members of the Fantastic Four, accidentally bombarded by cosmic rays, gained power to stretch his body, husband of Susan Richards, father of Franklin Richards|
There are multiple individuals who have acted as imposters using this name.
There are multiple individuals who have used this name in alternate realities.
Mister Fantastic (Earth-9032) - Johnny Strom, took his powers public and used them to entertain people