As far as the best US sites for online poker, you already have the top 3 with ACR, Ignition, and BetOnline.
I am also in the US and am constantly on the look for new poker sites available to us, the only problem with new sites or smaller sites is that you run the risk of not being able to withdraw your funds. For example, Intertops and Juicy Stakes poker can be played online in the US, but I have seen many complaints about not being able to withdraw if your address is US-based. Some other sites I've tried include Global Poker and 64spades.
Global Poker was good for me until they changed their deposit and withdraw method. They used Paypal for a short time but had to switch over to Worldpay, and now I can neither deposit or withdraw via credit/debit card. I also found out that, because of the nature of the site(site claims all winnings are "sweepstakes winnings") that taxes must be payed on each individual win, and furthermore, for each game the winner is pre-selected, meaning the site already knows the outcome of each game. You can read more on that in their Terms of Service, but I wouldn't recommend the site anyway.
Basically, unless you live in New Jersey, Delware, Nevada, and now, Pennsylvania, you are pretty much limited to BetOnline, ACR, and Ignition. I wouldn't trust any new site that you see, until it has been out long enough to have customer review and feedback. I will not deposit on any site other than the three mentioned above. It sucks, but that's the current state of online poker in the USA.
