Obviously Alisson is number 1.
If I was given the choice between Karius and Mignolet as backup then I'd go with Karius all day long.
Both have been dreadful for Liverpool let's be honest but you have to be a certain type of goalkeeper for Liverpool and Karius has some of those traits where Mignolet doesn't.
Mignolet needs to be at a club like Burnley where the whole game plan is defensive. They pack two walls of four infront of the goal and protect the keeper. Mignolet needs this because he never takes responsibility and prefers his defenders to do so instead, he never commands his box or leaves his line, he can't play out from the back, distribution is poor, the list is endless.
Karius on the other hand does leave his line and act as a sweeper keeper. He can play a bit too. Put it this way I feel a lot more calmer when we pass the ball back to Karius than I do Mignolet. If I see Mignolet do another cruyff turn I think I'll die of a heart attack. Karius comes for things too and is happy to take responsibility away from the defenders. His distribution is okay and in particular he wants to release the ball quickly and get on the counter which was refreshing to see rather than watching Mignolet hold on to the ball for an hour waving everybody forward.
Karius confidence is just shot to pieces and it's a shame because I have seen parts of his game that I've been impressed with. I can see the potential. He's definitely more of a big club type of keeper than what Mignolet is anyway for the reasons listed above.
Karius is under the spotlight right now and is being heavily criticised (and rightly so) but anybody who prefers Mignolet over him has a very short memory in my opinion. Simon Mignolet is the worst goalkeeper I've ever seen at Anfield.