In Final Fantasy VII, the summon materia that revives fallen party members is Phoenix.
Phoenix is a summon materia associated with the legendary firebird of the same name. When summoned, Phoenix attacks all enemies with a powerful flame-based ability and then restores the party, reviving characters who have been knocked out. This combination makes it unusually valuable: unlike most summons, its effect can both damage opponents and recover the team.
In the original Final Fantasy VII, Phoenix materia is obtained through the enormous Materia Cave side quest near Fort Condor. It is linked to the Fort Condor storyline, where the player helps residents defend their settlement from Shinra forces. The materia is also notable because it can be paired with the Final Attack materia, allowing Phoenix to activate automatically when its holder is defeated.
The other listed summons do not perform this same party-wide revival role. Alexander uses holy-element attacks, Odin is associated with instant-death or slicing attacks, and Ifrit uses fire damage. Phoenix is therefore the canonical answer for this question.