I was never a big fan of Ramos and I fear he won't get into my favourite players bracket anytime soon either, but credit where its due, he's proven to be a very good CB since he left the RB position, as well as recapture his partly lost aura as a massive goal threat in set pieces. He may be not the most intelligent player out there, but his commitment and passion are there for all to see, and quality-wise he doesn't have to envy anyone.
As for the top 5 CBs in the world, of course form and luck always play their part in football and can easily turn heroes into villains and viceversa in just a few weeks, but I would say that Pique and Ramos have to be up there if only because of their skill, hierarchy and experience. If they are in tune I cannot think of any couple being better for a first-tier competition