I find the the Hilton ORD is the most convenient (due to location of being connected to the main terminals and very close to the Blue Line train to downtown). It has pretty good facilities. It also has reasonably good Gold and Diamond treatment. The downside is parking costs more, but I don't know if that is an issue for you or not.
And if you want to save money, the Hilton ORD shows up on Priceline for very cheap ... but then you are technically not entitled to Gold benefits. But I know that my mileage with that really varies.
