it depends---are you talking about for piecing or for quilting? or both?
for piecing i would choose a neutral that wont really show up anywhere-
for quilting it again depends-
do you want the thread to show?
or would you rather it disappear?
you could use invisible thread- or a medium gray
if you want it to stand out then a gold- or a different stand out thread-
i usually pull a couple different ones i think might work- pull some thread out and lay it across the quilt top to see how it looks on the fabric
there are also nice verigated threads on the market that could be an option- again- if you want the stitches to show up