I'm considering a large fish for my aquarium - well, not really large, but center piece sized. What do you reckon would be a good fit for a 90 gallon?

Why don't you look through some pictures of saltwater fish and see which catch your eye? Or look at guides of fish for the beginner/intermediate/expert saltwater keeper (whichever level of expertise you feel you're at).

Personally I like toby puffer, flame hawkfish, and purple tangs. I don't know if these are the size range you're looking for with a 90gal but they're all beautiful and easy enough to keep.