Darksteel is clearly the most powerful because of two cards that would change type 2 (and shape the future of extended) forever:
Fifth Dawn has a couple of good cards (Eternal Witness and Rude Awakening), but they weren't nearly as ridiculous as Skullclamp and Ravager during Regionals.
Mirrodin has to come in a close 2nd because it made the extended format that year ridiculously fast and caused the Japanese to use Gilded lotus in Twiddle Desire. Thus, the deck today was caused by Mirrodin. Second, the Tinker Stax deck (President Bosh) was simple ridiculous and made the format last only a few turns. Thus, we see a load of bannings.
Hands down one of the better sets we've seen in awhile.
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Mirrodin for sure. Of the three, it had the highest average power level and the most innovative cards. I really loved this set.
Darksteel had some really (possibily too) powerful cards, but it also had a higher jank percentage (at least, that's what I've noticed). And, as for Fifth Dawn, it was pretty meh. I didn't like Sunburst at all, and there weren't many good cards, either.
Im not so sure that mirrodin had the highest power level, it had a lot of good and interesting cards, but darksteel gave us the most broken cards i think. before darksteel, mirrodin block was balanced and interesting, with affinity being good, but not broken. that being said i still really like mirrodin as the best set out of the three, for the reasons i stated above.