Although the "Four Kings" is typically the accepted moniker for those four fighters, I've always felt it to be a little of an affront to Wilfredo Benitez, who faced three of the four, and beat one of them (Duran).
The Leonard-Benitez fight was quite competitive, and I thought was stopped prematurely, although Leonard would've still been the clear winner by decision.
Wilfredo defeated Duran by UD in 1982, and then lost to Hearns by MD in that same year.

Regardless, since the OP asks to rank the Four Kings, I'll do so.

I rate Leonard as possibly TBE. So he would be #1.
I feel Duran had an incredible career, spanning various weight divisions. He also fought and beat many ATG's. So he'd be my #2.
Between Hagler and Hearns it's tough, as I liked them both.
But I'll go with the head-to-head and pick Hagler.

I'm reluctant to put Hearns 4th, as he was also a favorite of mine.
But it would be:

1. Leonard
2. Duran
3. Hagler
4. Hearns