Which Super would be the most powerful...

If they possessed all the various powers they've had at one point or another simultaneously (and could use them effectively in that state)?

So for example, Hal Jordan would have his usual Green Lantern ring, the rings of pretty much the entire Lantern corps that he stole during Emerald Twilight, the Blue Lantern ring, the Sinestro Corps ring, the Red Lantern ring, the White Lantern Ring, the Agent Orange powers, the Parallax entity, the Spectre's powers, and whatever else I'm forgetting.

Thor would have Mjolnir and his usual suite of powers, along with the Odinforce, the Thor Force/Rune King powers, all the weird enchantments Mjolnir used to have (casual time travel, multiversal portals, turning into a powerless human >.>)...

From my (broad but admittedly limited) knowledge, Superman might have a decent shot at claiming the title, partly just because of his sheer length of publication history, how utterly bug-**** insane his Silver Age powers were (both the sheer lunatic scale of his usual ones and the bonkers random ones he got from red Kryptonite or whatever other plot device), his brief, unfortunate bout with energy abilities, and the final clincher of the quasi-canon Prime Superman of DC 1,000,000, who apparently had 'all the powers of the Superman Dynasty', which included still more high-grade energy manipulation and Fifth-Dimensional-Imp magic, which is reality-warping on a truly absurd scale.

Any other characters that have an insane collection of powerups scattered through their history that could compete with Electric-Prime-Silver-Age-Superman or Rune-King-Odinforce-Thor?

Thanos had the Cosmic Cube, the Infinity Gauntlet and the Heart of the Universe. He can turn any other guy into dust.

The correct answer, obviously, is Squirrel Girl.

The Heart of the Universe wasn't canon, was it? If it is, yeah he wins. If not, he just loses to Doctor Doom during Battleworld, where he managed to out-muscle the Infinity Gauntlet on its own field