Toss up between India and China ?
History has a tendency of repeating itself, and while the US will never lose the accolade of the Apollo missions (first men on the moon) it's not impossible to see the US in a similar "we've fallen behind" shock that Sputnik engendered.
If I were to place a bet on the next set of boots on the Moon it would be China, followed by (or in partnership with) India.