it depends on what generation and type of music your into...
the 7 divas concert show cases the older generations which for me are the everlasting greatest singers out there... for me i dont think the younger generation singers can compete with the older generation whether its male or female.
girl rockers (not hard core rockers)... older generation: amp, au, mai ... middle generation... parn (although im not a fan of her or her songs but her voice is good) da endorphine, panadda... i cant think of any young generation rockers ... more on the punk rockish side theres no more tear
theres also Fahrenheit, Klear, and Prik Thai (personally i like Fahrenheit a little more than the other two but they aren't some of my favs..)
older generation singers: christina, sarunya, maleewan, nantida
middle generation singers: beau sunita, nat myria (her more recent stuff i didn't think she was that great back when she first started, look for things after her divorce from tao ahha), lydia (if you really like that nasally r&b sound) mod (3g) from grammy for that same type of sound that lydia has, punch
young generation singers: gam the star, den mic idol.... this is also hard to name some newbie singers with really great amazing voices
then there are also of course the overload of sugary pop music that is more just for non serious listening and not if your looking for good voices...
but if you want to get back into thai music... check out
DaraThai