I think Matip and Van Dijk is the best partnership. In fact I think Matip would be first choice if he weren't injured. Not a slight on Gomez, but Matip is that good.
I've previously said that Gomez with his pace takes our already excellent defence to an even higher level, but right now I'm not sure about who I prefer. Both are superb at their best, but I think Matip is probably better suited to lead the defence, so if I had to chose one when Van Dijk is unavailable it would probably be Matip. Both are excellent at their best though, just a shame niether has any luck with injuries.
It's also fantastic how well Fabinho's adapted to playing in defence. You'd think he'd played there all his life, his reading of the game is outstanding. He's not very similar in terms of style, but it's like having Desially in our side, world class in both defensive midfield and at centre back.