NOT most/least powerful.
Best, I personally think block stands out more than any other as what I'd like to see in future games. Other than the 5 point sink at the beginning, every perk changes how you can use a shield and adds utility rather than just making your character's stats raise.
Worst, I'd go with illusion. Part of it might be the fault of illusion spell design, but everything in the tree either makes the spells cost less or allows them to affect more creatures. Not an interesting perk in the entire tree, your use of the affected spells doesn't change, you just get to use them against more targets instead of having them essentially fizzle based on the target's level which you can't see. In fact having such a reference to creature levels is a bit immersion breaking, seeing the little message telling you a bandit _insert scaled adjective_ is too high level for calm or pacify or whatever is not so great - you're trying to into the gameworld/atmosphere and they suddenly remind you of all the hidden numbers in play with a not very subtle text message.
Edit: Quiet casting is cool actually. One decent perk in the tree. Still the worst though.