Sephiroth
If collateral damage isn’t a factor to consider, then Final Fantasy VII’s Sephiroth would be an ideal opponent for Saitama.
Host to super-human abilities thanks to his being exposed to cells from Jenova before birth, the former Soldier elite is a master with a sword, magic and hand to hand combat. He can also teleport to and away from his opponents seemingly at will, and can summon angels to drain an enemy’s health away from them in a flash.
And that’s just what he can do before he transforms. Should he choose to go all out, he’ll gain abilities that could only be called god-like, summoning meteors capable of destroying solar systems before making their devastating impact.
Should he and Saitama ever cross paths, he could whip out said meteor attack to wipe out the planet they’re fighting on, giving him a solid minute or so where Saitama wouldn’t be able to breathe due to being in the vacuum of space.
Things’ll still be touch and go after that, but it would at least even the playing field enough for Sephiroth to get some hits in before suffering the damage from one of Saitama’s punches.
