The one that made you frustrated, confused, or just laugh at how broken the system can be.
Drop it below 
Let’s see how bad (or absurd) it really gets.
I’ve mostly had great experiences, honestly, but one time I went in for support and they had to call the support line. I was stuck standing in front of the desk, listening to them navigate the support line.
Runner up — I was told when I moved to Australia that I should bring all my money as a bank draft. The Aussie bank couldn’t cash it for some reason. Had zero money in a foreign country for days while I called and begged them to reverse the draft so I could use Wise instead.
I think my most recent one was, we had a little bit of a winfall and ended up getting a nice cheque from it, took it into the bank figuring that would be the best and fastest way to access the funds since it was from a government agency and after they had deposited the cashier told me that we would have had more instant access to it if we’d put it in the machine 
Would have been great to have been told before and make a decision from there.
Classic haha they need to wait for someone to come by and check the cheque machine. Some of the older machines aren’t totally automated.
I recently had my bank put a hold on a bank draft I deposited with a teller; I wonder if I had the same/similar issue. The hold seemed odd to me as I had deposited a bank draft for a similar amount, from the same estate account (inheritance), also with a teller about a year prior without issues. 