My take on the whole force situation is that the most powerful force users to ever exist were Darth Plagueis and Yoda. Anakin Skywalker would have been next in line and potentially stronger than both, had he been able to fully realize his complete potential; but losing his arms and legs somewhat muted his ability. Even if he remained very powerful as Vader.
Luke continued the family line but could only have been more powerful than Vader because he kept all his limbs (minus hand). Now bear in mind that all the Jedi are dead. So his exposure to other force users would have been largely limited to those kids he trained. Of those Kyle and Rey are the most powerful he’s seen, but I don’t think either are anywhere near as powerful as previous greats such as Plagueis, Palpatins, Yoda, Windu, Vader etc.
I also think it’s very likely that to reach your full potential in the force takes a very long time, and a level of insight and wisdom that only comes with age. The most powerful force users we’ve seen have typically been ancient. On screen the most powerful force user we’ve seen is probably Snoke. And it seems extremely likely to me that he’s probably not dead, with that being an illusion for the sake of Kylo Ren. He is also probably Plagueis.