I guess that the best option for you, as a US citizen, is coinbase. Although, I'm not a frequent coinbase user (am not a US citizen but i used coinbase for store in the past) but I think they are still one of the most reputable availabe services in the crypto market.
In my opinion, it is the number one choice for ACH transfers with a friendly-use interface. Note that the fees are relatively high for all the services it provides. You can overcome the deposit/withdrawal fees for ACH transfer by using the pro version of coinbase called GDAX; the user interface is a little bit hard to understand for a beginners, as it's targeted to the more experienced users of coinbase.

The problem with Gemini is that it's not available, AFAIK, in all the US states, so this option is only viable if you are in a state that they operate in.

OP, please move the topic to Trading Discussion board.