Depends on what you mean by powerful?
If it's destructive power, measured by the scale of damage it can cause, then giant summonings, great fireball technique, chidori, rasengan,amaterasu or even Tsuande's punch are on the list...probably more could be listed (like Edo Hashirama's forest jutsu, Enma's transformation, or some of Kimimaro's and Gaara's techniques).
If you mean a jutsu that can one-shot the enemy, especially in a 1 vs 1 fight, then tsukuyomi or gentle fist can also be mentioned, as well as Jiraiya's toad stomach summon, but Shikamaru's dad's or Ino's dad's jutsus soloed fodders too. The potential of Kisame's Samehada was also hinted against Asuma. Oh and there is the Shiki Fujin. I guess Gai's and Lee's gates can be also counted as op in part 1.