I'm asking for the biggest power growth in a short time, of course.
Game protagonists are definite mentions. The Skywalkers, particularly Anakin, Luke and Cade, are all worth mentioning, of course. Savage would be another case - I think Ventress and perhaps even Obi-Wan are also worthy of mention.
I would think Sidious is as well, given that he mastered lightsaber combat in just a few years and his powers were advancing at insane rates in Darth Plagueis.
Vitiate and Nihilus likely grew greatly as well as a result of their planetary devastations, but those were more one-off events boosting their powers as opposed to actual growth.
If we don't include Oneness moments/temporary boosts or recoveries from injury:
Originally posted by SunRazer
I'm asking for the biggest power growth in a short time, of course.
In a really short time, it would be pre-post Nathema Vitiate, pre-post Plagueis death Palpatine, and maybe ditto with pre-post Force unbalancing Plagueis/Palpatine.
For a few weeks, it would be pre-post Yoda training Luke (or was that longer? It should be much longer given that the Falcon was not using hyperdrive, but that's not what the timeline says...) and maybe some of the video game characters like Katarn. Does Revan post-amnesia count?
In the span of a few months, it would be ESB - RotJ Luke and TCW - RotS Anakin.
For a few years, it would be AotC -> RotS Anakin, RotJ -> DE Luke (or Luke in one of his other periods like NJO), and maybe AotC -> RotS Obi Wan. The Outlander might also get a nod.
For the period of decades, it would clearly be Luke.
So, let's see here:
Anakin grew from roughly 80% of RotS Sidious to 100% during RotS.
Vitiate went from a nigh-featless Sith Lord who could dominate adults' minds, use Consume Essence, and Force Sever planet-scale Sith to a godlike avatar of the dark side capable of warping the atmosphere and Force alignment of planets, freezing lightsabers mid-swing, and frying 12 planet-scale Sith at once (still weaker than whatever incarnation of Sidious that quote from The Complete Visual Dictionary describes).
In other words, Minax-tier (or Marr-tier at best) to nigh Plagueis-tier.
Luke went from TESB to RotJ in a year.
Sidious went from Plagueis-tier (certainly above Vader and Caedus) to well above that. Not much relatively speaking, but it's something.