Adam is the only one who has shown potentially outstanding vocals, but it's early in the season, and one of the others could stand out with the right songs and chance to perform.
Alisson and Alexis both made the mistake of singing songs they just don't have the vocal ability to handle, highlighting their weaknesses. Based on the things they showed themselves to lack by doing this, I am leaning toward not expecting much from them, but haven't written them off completely yet.Both have reasonably good voices, and there aren't any standout female singers so far this season that have been put through to the live shows, so with the right songs and strong performances, either may be around a while anyway.
The rest of them haven't made an impression at all, though the producers seem to want Danny in the top 10 /possibly the final. I have been disapointed with the groups shows so far.