1. A'Sharad Hett - Krayt - this one is obvious. Hett went from somebody near AotC Anakin at best to... well, Krayt level, as a duelist. From a guy with nearly no Force feats more impressive than some shoves on Aurra Sing, to the guy who DT's Cade Skywalker, stalls Karness Muur with Lightning, shoves off hordes of rakghouls, etc. etc.
2. Revan - Mandalorian Wars Revan was a great warrior, definitely, but his best feats surround Yusanis and Mandalore. Impressive, no doubt, no smallest doubt, but that's something to expect from any good, powerful Jedi/Sith. By SOR we had a completely terrifying monster, and that was a mentally damaged Revan with missing parts of himself. Actually, Revan could be the 1st place, but I would put Krayt's development higher.
3. Palpatine - slightly ahead of Marek, and still debatable. After all, as of TPM he was around Plagueis level, and DE Palpatine isn't so hugely more powerful; he got far more masterful, carrying his super storms. While he got more powerful by a large scale, I hold my doubts mainly because rating Galen's advancement in the Force is hard; he got two leagues greater as a duelist, but TFU shows Force... differently. So I can't really compare Palp's and Marek's development in Force power. For Palpatine, definitely not much of a change as a duelist... but you talk about "power" here, so not sure if I should include this too much. If I should, then Galen's advancement as a combatant makes Galen's advancement as a whole > Palpatine's advancement.
4. Galen Marek - above.