It seems like Gemini is the next up-and-coming contender. I'm not sure whether they are competitive with Coinbase's volume yet, but of the newcomers I know in the space, more are using Gemini than anywhere else.
I tried to sign up with Gemini, but unfortunately, they only accept IDs that comply with REAL ID. My state doesn't comply yet (I assume they will start complying by 2018 due to new federal transportation regulations). I wish they told me that before rejecting my documents, because I gave them a lot of really sensitive info. I guess I either need to shell out for a passport (not really necessary for me) or wait until I can get a compliant state ID.