You'd have to be delusional or just ignorant to think Tsunade's bond with Sakura is stronger than Naruto's.
And where are people getting off claiming Sakura and Tsunade spent countless time together? Sakura was Tsunade's apprentice in training, not her assistant. Tsunade would occasionally watch over Sakura's training but most of the time was doing her duties as Hokage. They're not like Jiriya and Naruto who literally spent every moment with each other and were like Father and Son.
Naruto's bond with Tsunade is deeper than anything she has with Sakura. Tsunade literally sees her deceased lover and little brother in him and feels as if Naruto inherited their wills and is in essence, Nawaki and Dan reincarnated. She placed all her bets on him and if it wasn't for Naruto, she wouldn't have even taken up the job as being Hokage.
Tsunade and Sakura may have spent more time together training and what not, but Naruto and Tsunade have a tighter bond and are like family.