It's not the largest, and also not the 'most amazing' option. Poloniex was like that for a good while, but they messed things up big time, which resulted in an outflow of users jumping right into Bittrex, to yet again face the same issues they left Poloniex for. Bitfinex has started with slowly accepting new users again, but people first have to check certain boxes that they agree with everything before they can proceed with registering. I actually preferred them to remain closed for new registrants since I don't think anyone should sign up at that exchange after everything that happened. It's the largest USD exchange based on volume, but they don't even allow anyone from the USA to use their platform.