I dunno. Imp Init is useful for high dex fighters, low dex clerics, and just about anyone else that needs to act first (SA, buff spells, Group-nuke spells).
Power attack requires you to be strong and hit stuff. Good for fighters and clerics, bad for rogues and wizards.
I was going to vote II off, just because I thought Power Attack would take it all, but your arguments have swayed me. I have never had a character take PA, though I do see its usefulness (I don't usually play heavy hitters), but I often take II. So, regretfully: good-bye Power Attack.
Power Attack must go. Improved Initiative was my guess as the overall winner as one of the few feats that can be taken by anyone, is useful to anyone that takes it, and is actually worth a feat. If power attack had no prerequisite and applied to spells, it might be able to compete. But Improved Initiative would still win based on its sheer simplicity. Power Attack 3.5 is a mess.