a) emperor angelfish has already been mentioned - another one of my favorites is Australian harlequin tuskfish
b) Why? colorful, nice (but not too big) size, great personality, unique, a showpiece fish, cool behaviors
c) Is it reef safe? yes, except for shrimps, crabs, etc. mine is okay with larger hermit crabs and hasn't bothered the few snails in the tank
d) It it aggressive? not at all. it looks menacing, with blue pointy teeth, and a husky build, but it is very peaceful. it can stand up to moderately aggressive fish such as large angels, puffers, the more peaceful triggers, but is not at all an aggressor. I had one with a smallish tomato clown and a royal gramma for years, and it never bothered them. I'd be cautious with really small fish.