That's a big question....LOL
+1 on the royal gramma, just be certain you get a gramma and not a bi-color dottyback. They have very similar body shape and coloring, but the bi-color has a definite line separating the colors and on a gramma the colors blend into one another.
Bi-color dottybacks are VERY aggressive, so you don't want to make that mistake...LOL
A blennie would work as long as you keep in mind it's mature size and get one suited to your tank.
A dwarf angel could work, but as mentioned your tank may be too small and could elicit territorial aggression from the angel. But I suggest you wait on attempting one until your tank is at least 6 months along stable.
What are the actual dimensions of the tank because that could also be a determining factor. A long rectangular tank opens more possibilities than say a cube or hexagon.
Stay clear of tangs until your tank is very, very well established and stable.
May I suggest taking a peek here if you haven't already;
Beginner Fish: Tropical Fish for Beginners in Saltwater Aquariums
(I hate going to that site...........it always makes me want to set-up more tanks... )