Supermale lineatus fairy wrasse is pretty awesome, Mata puffers are pretty amazing, Red Sea regal angels are gorgeous, I can't even decide so many awesome choices
I couple months ago I got 7 Zebra Bar Dartfish... I think they're cool because they are hands down the most entertaining fish in the tank.
All 7 sleep in a cave in my rock at night, but in the morning one brave soul comes out and scouts the situation. After about a minute the other 6 come out one-by-one spaced a couple seconds apart. The all hang together in a shoal all day. They dart about and swim in all areas of the tank. But when one of the bigger fish make a sudden move, all 7 vanish instantly. Then the whole process starts over again.
They acclimated quickly, eat anything, and are very easy to care for. They're just really fun and entertaining to watch.