Carlsen's current live rating is once again higher than Kasparov's highest ever rating. The pack in #2-15 is less than 50 rating points apart. Carlsen is more than 50 rating points ahead of the pack.
I know, there will be people who say that ratings are inflated. There are fewer players rated +2700 than there were a few years ago. Statisticians such as Kenneth Regan have found no evidence of rating inflation.
Kasparov is the only player in history who could have given Carlsen a consistent challenge, and he probably would have lost in the long run too.
Yes, strongest ever only Kasparov might compete with Carlsen for, I think.