To be honest I would register with all the main exchanges if you are serious about crypto, some (New) coins/assets are not traded on all the exchanges.

Top Exchanges:
Bitfines
Bithumb
Coineone
Bittrex
Polonix
Kraken

For example it is possible to process deposit within one day with Kraken.com, once you are in Crypto you can withdraw/transfer it between exchanges as you please for the future withdraw via exchange of your choice, but I know some people that have been involved with crypto for 1-2 years and newer had the need to withdraw any of their funds into their bank account. I would worry more about deposit processing, but that is me.