They're both extremely talented.
They both have different strengths.
How you can quantify which talent is greater boggles my mind. That's like saying who's the greatest artist Da Vinci or Rembrandt. A totally subjective opinion with no basis in logic. People simply like what they like.
This would be similar to saying who's more talented, Gasquet or Monfils? And then saying Gasquet because his game looks more fluid in your opinion. That still doesn't answer the question of talent. It simply shows your preferences.
But, if I had to pick I'd say Nadal:
Playing with your non-dominant hand and being #1 in the world twice and amassing 8 major titles is unheard of.
Also, his shots are unbelievable, maybe not as much variety, but definitely more effectiveness.
He also makes less errors, has no weaknesses, and doesn't shank many balls.
His footwork is astounding and his on-court strategy incomprehensible.
His ability to adapt unparalleled and the notion that there's still room to improve, commendable.
His humilty is an added bonus.
Tennis is a sport, not a dance recital. In sport I want to see a warrior, not a dancer, so maybe that's why I'm not impressed.
Monfils vs. Gasquet?
I can't say really, they both have talent, but I don't get into Gasquet's game either. Flashy, but ineffective. I hardly rate his talent as mega, since it takes many more components to equal talent.