Those D are not comparable. What type of D are you looking for? Saying best doesn't help clarify the actual need if you're trying to to replace Ceci. You don't replace a Ceci with Karlsson, because Ceci was never intended to have that type of role. So what are you looking for?
We need less of a generalist than Ceci - more of a specialist. It doesn't matter if it is an offensive specialist, or a defensive specialist. I believe that the issue right now, is that Nurse needs to focus on either being the smooth skating, puck moving force that he can be, or do less of that and focus on being a crease clearing defensive defenseman. The problem is, that Ceci doesn't do any of those things - which is fine in the right role, but the Oilers put Nurse out against the opposition's top players all the time, so having Ceci, a guy best suited to play as a #4 (or bottom pair), who does everything pretty well, but nothing at a top tier, just isn't the right fit there. So, the Oilers just need a specialist to play with Nurse - it could be a Karlsson type, that would allow Nurse to focus on the defensive side of the game, or a defensive specialist, so Nurse can rush the puck, and we have someone to cover for the mental lapses that happen when he spends so much time joining the rush.
Personally, I hope its a guy with more on the defensive side that we go after, so no Karlsson, no Dumba, no Klingberg, no Shattenkirk. That leaves Mayfield or Severson from the OP's list, although I would prefer a guy with some more sandpaper and snarl to his game to fill the spot next to Nurse. Man, I miss Adam Larsson, he was a great fit there. I thought Ceci could fill what Larsson had left when he went to Seattle, but he doesn't quite get there in my opinion.