As most of you seem to know Novak's running FH while good is not a great shot. Nadal's OTOH is, but I'd still put it behind Pete's because of its predictability. Though I don't claim to have done any systematic analysis of his running FHs I'd wager that at least 80% of the time Rafa chooses to go DTL. Now that's no doubt due to his heavy topspin which gives him lots of margin for error, but I suspect the main reason is that SW grip of his which makes it relatively hard for him to generate pace on the run. (Pete like most preferred to go CC, but not nearly as much.) Since today's players don't come to the net as their main tactic Rafa can generally get away with it, but good net players would've cut off the angle and made him pay. That's why I can't place Rafa's running FH on par with Pete's and Lendl's for that matter, though I agree it's in the best-of conversation.
And speaking of who Lendl is the only one I think can be said to be superior to Pete here, and several knowledgeable posters whose opinion I respect have said as much. But I still go with Pete, for this reason provided by among others
@slice serve ace and Datacipher (who has nothing whatsoever to do with a certain barely active poster): Nobody likely before and certainly since has "halved" the court with his running FH quite to the same extent as did Sampras. As you may recall Pete especially in his later years used his CC BH almost purely as a rally shot, with lots of topspin which meant it often fell short. But few players were much willing to punish him by hitting it back DTL. The reason? You guessed it: that DTL response had to be inch-perfect, because otherwise there was a good chance Pete would take immediate control of the point if not win it outright with that running FH, as in the first point of this TB:
Which is why people make a mistake when they knock the Sampras BH for not being a point-ending weapon. All those "moonballs" were in fact a very conscious choice on Pete's part, and while it can be debated whether he made the right call (his former trainer Etcheberry for one thought it hurt him in the long run) what's undeniable is that this unusual game plan wouldn't have been possible without his trademark running FH. Overall I'd say Pete, Ivan and Rafa as the three best in this category of the past 30 years or so, in that order.
Also Fed isn't quite up there with this trio. As I believe
@jrepac once put it Fed's running FH is more a placement shot than a power shot. Even in his prime it'd be singled out as the one weak link in an otherwise flawless weapon. It doesn't necessarily prevent his FH from being the best over, much like Pete's having a relatively weak BH smash doesn't mean his overhead overall isn't among the very best, but it's a weakness nonetheless.
Speaking of which we all know about Murray's problems with his FH, but I think his running FH is top-notch and in terms of sheer pace it may well supersede Nadal's. In fact I'd probably rank Andy only second to Rafa among the Big 4 here.