You would need to define exactly by what you mean as "Who is the most powerful force user?" As I've seen mentioned above, there is a myriad of ranges when it comes to the Force and not one single person is a master of all of them. Thus not all of them are truly the "Most Powerful." Just because you can throw around Lightning all day doesn't make you powerful if you're too blind to see the dagger at your back.
There are powerful force users yes, however even the greatest ones; I'll use Yoda and Mace as an example, can be claimed to be the most powerful. Yoda lived for 900 years and he still wasn't able to see clearly as to whom Sidious was or how the Sith were returning. Mace Windu was the creator and a master of Juyo and even that mastery didn't mean he was the most powerful when he was distracted by facing down Sidious and not thinking that he should also be watching Anakin.
I suppose the best way to look at it would be by looking at Form VI, Niman. It is a jack of trades, master of none. That where it has its power, and at the same time its weakness. It's not meant for prolonged combat, it even says that all practitioners whom used it on Geonosis were killed. Form II, Makashi is designed solely for lightsaber on lightsaber combat where its mastery lies, though fails when it comes to fighting against an opponent with a blaster.
To claim to be the "Most Powerful" doesnt serve more than to stroke ones ego. You can be a master at a specific skill, or a practice, though that doesnt mean you're the "Most Powerful" out there.