I think a must-have fish, if anything, in a marine tank is a yellow tang. That's assuming you have a 55-gallon tank or larger. They've always been a favourite of mine.
I recently picked up an incredibly shy flame angel. Everyone always picks him as their favorite fish because of his bright colors.
I also have a zebra lion in the same tank. He has bright red pelvic, dorsal, and anal fins. His pectoral fins are a bright green with two blue spots. He's absolutely gorgeous when he swims, which I've only seen enough times to count on one hand Usually he drifts through the current upside down. Yesterday I managed to get him to swim backwards upside down while I was chasing him with some food lol.