I saw a "what if" where Phoenix fought Galactus to a draw.
Galactus is not invincible. The Beyonder swatted him like a fly and then later Doctor Doom absorbed all his power to fight the Beyonder. This was in the Secret Wars mini-series.
I think Galactus helped fight Thanos when he had the Infinity Gauntlet. Thanos beat everyone there. EVERYONE. The whole Marvel universe (literally - including Eternity) ganged up on Thanos and Thanos won due to the Infinity Gauntlet. At that moment he was the most powerful being in the Marvel universe. He was more powerful than Green Lantern when he became Parallax.
I don't think the DC universe ever had anyone that powerful.