Ramos has been poor this season..An extremely error prone center back although does possess good ball playing skills..Also he's a very tempermental defender not as much as Pepe but still a negative attribute for me.
Also Hummels possesses a great passing range..His long balls are often accurate..Bonucci's passing has improved only from this season onwards.
Ramos is best as a full back not as a CB. If anything, I see the biggest reason for Mou's troubles with Madrid as their very poor defense. They are all good going forward but defensively they really suck. Compare that to Inter's back four under mou or chelsea's or even porto's and you'll see the gap. Even in terms of DMs, from Essien and cambiasso to an old rusty essien and a khedira.
Ramos has really sucked this season, I don't usually see R. Madrid games but I've done it like 8 times this season, and Ramos has been terrible, specially in La Liga, I remember seeing the game vs. Sevilla and he was the worst on the pitch, and then people asked why he went to bench the next game.