Hash tables don't have a top. I suggest adding actual entries as numerical indices and have some other keys (current, next, last) which point to the indices.
hinc playlist last
hadd playlist $hget(playlist,last) $2
if ($hget(playlist,current)) hdel playlist $v1
hinc playlist current
echo -ag current song is $hget(playlist,$hget(playlist,current))
All that said, appending to a plain text file then reading and deleting the first line would not be a terrible implementation for this either.