I’ve had a bad experience with 3 banks - Virgin Money, Starling, and Lloyds, both of which were in the process of opening my account which I subsequently decided to stop for both banks. Starling’s customer service seemed to be completely non-existent and had to spend ages on the phone waiting for them to pick up and then the attitude of the Lloyds’ agent annoyed me quite a bit. I was able to contact Virgin Money very easily, however, they were completely useless.
My other banks that I have experienced the customer service for (Monzo, Barclays, and Nationwide) have been very good.
I also have a Santander and Tesco Clubcard Plus account with Tesco Bank but I’ve never needed customer service.
I have subsequently cut down on the number of accounts I have. I now only have accounts with Santander, Nationwide, and Tesco Clubcard Plus.