i'm limited to what i know (compare to everyone else lol) but i'd say Thanos greatest show of will was his final confrontation with the "cosmic conflagration" specifically with Eternity during the infinity war. The panel showed what seems to be him willing himself to a win over Eternity. Then there's the fight with Tyrant and Odin - and i'm pretty sure there are more, i just don't know them lol. Doom's best showing was after amping off of Galactus's ship in Secret War he fought his way to the Beyonder ultimately usurping his power, ha, both had feats that were pretty crazy but i think i will give this to Doom based on scope of confrontation.