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.