"Most powerful" can mean different things depending on the build, and how much work you're willing to put in to exploit it. Greenbound Summoning is probably the simplest feat to abuse. Leadership can give you a lot more power, but will require effort on the part of the player (and help from the DM).
Improved Combat Expertise, from 3.0's Sword and Fist (and a number of other sources), removed the +/- 5 restriction from Combat Expertise (now just Expertise). Not amazingly broken in the "kill them and take their stuff" way, but by far the most powerful feat there is if you're trying to maximize AC. Combine with All Power Attack All the Time for best results.
The unerrataed 3.0 Quicker than the Eye (from Song and Silence) granted you a free standard action if you threatened an enemy that was denied their dex bonus to AC. This meant that if you carried a blind kobold in your backpack, you got the effect of a continual 3.0 Haste. Very nasty. It was later changed to only grant a free attack against the enemy who was denied their dex bonus.
Divine Metamagic from Complete Divine can be extremely powerful if you work for it. It allows a cleric to burn turning attempts to use metamagic feats instead of using a higher spell slot. The problem is that it doesn't limit the maximum spell level equivalent to the maximum spell level you can cast, which opens up access to things like Persistent Divine Might. There were a number of psionic feats that had this problem as well, but IIRC they were all fixed in errata.