Which fictional cosmic entity that consumes planets is the longtime Fantastic Four foe?
Superman/Fantastic Four:  The tale unites DC's first superhero Superman and Marvel's first superhero team, the Fantastic Four—pitting them against the Cyborg Superman (who gained notoriety in 1993 during DC's "Reign of the Supermen" story arc) and the longtime FF foe Galactus. Galactus: Galactus ( ) is a fictional character appearing in American comic books published by Marvel Comics. Galactus:  Formerly a mortal man, Galactus is a cosmic entity who originally consumed planets to sustain his life force, and serves a functional role in the upkeep of the primary Marvel continuity.
Galactus