A variation on the usual versus threads, the question is what is the most powerful version of a particular Marvel character depicted so far.
Alternate realities and future versions of the character are not to be considered.
Iron Man (Avengers vol 8, issue 5, Sept 2018): Godkiller mark II armor - a Starktech giant robot adapted from a Celestial super weapon. Despite matching their size, the armor was quickly destroyed by the Celestials as the limited earthly technology could not begin to match them in power.
Hulk (World War Hulk, issue 4, Nov 2007) Worldbreaker Hulk: fueled by betrayal and personal loss, the Hulk reaches a point of unrivaled anger and thus strength, to the extent his very strength threatened to devastate the Earth.
Thor (Thor vol 2, issue 80, Aug 2004) Rune King Thor: gains not only the full Odinforce, but also mastery of rune magic on a level beyond that of any other Asgardian.
Spider-Man (Spectacular Spider-Man, vol 1, Issue 158, Dec 1989) Cosmic Spider-Man: possessed temporarily by the cosmic Uni-power his strength is increased a hundredfold and gains flight, cosmic awareness and the power of energy manipulation.
Colossus (Uncanny X-Men Vol 1, issue 542, Aug 2011) Avatar of Cyttorak: in order to save his sister, Piotr gains the powers of the Juggernaut.