Well personally I never head a bad experience with any of exchange but if I do the leaser search then I will say with Bitfinex I have had a bad experience but I will not blame them for this. After moving some of the token I figured this is not even the minimum required token for trading. So if I log in there then possibly I will see then there still but I do not bother much.
I have been with Poloniex, Bittrex, Bitfinex of course, exmo, gate.io, Livercoin and many more exchanges I can not remember much. But finally I figured I am regular with Kucoin and Binance.
Not sure comparing with you lot how is my experience or exploring numbers but I am sure people have different experience with different exchanges. May be we will talk about good experience in some other topic but in here I am really interested to know your worse or unpleasent experience that happened with you. May be we will be able to some lesson from your experience.