I don't have any experience with flowerhorns, but I feel like you'd need just a tad more space for one. I think it's a close one, maybe someone with experience can chime in here.
When you say "a single fish with a lot of personality" Oscars come to mind. 1 oscar would go great in a 55 and he can live his life there. They grow very fast and have a great personality. They are one of my favorite fish.
Red devil can be insanely aggressive to other fish and even people walking by. A big red devil will literally make that tank its own.
Green terror can be cool. I have not had luck with them. In my experience, they go anywhere from terrified of everything to mass murderers like red devils. And although a stunning fish, they grow very slow compared to other similar cichlids.
When you say aquascaping, are you referring to setting live plants up? or just rocks and other things? Pretty much any large cichlid is going to dig and arrange the tank to it's liking and the end result will be the destruction of live plants.
If you are aiming to get the most out of your aquarium with just one fish (depending on the species) you are best with a simple setup to maximize swimming space.
I have a lot of experience with that tank size, and I can't think of any other fish that I would keep by itself in that tank size than an oscar. Maybe I'd play around with the idea of a red devil.
