Lionfish would be sweet, but can be dificult to keep so if your just begining, it might not be the best fish for you. I would go with a grouper, a panther grouper would be cool, or maybe even a sea robin. 

Dude you can put more than one aggresive fish in a 90 gallon, and as for lionfish being territorial and aggresive not really there actually pretty docile. Now this is an aquired taste of mine but I would put 4 or 5 squirrelfish. You wanna talk about something being dead before it even touches the water nothin beats these guys. but if you really only want one fish that is the shiz. I would have to recomend.
1. Clown Triggerfish(Large)
2. Blue ribbon eel(very hard though!)
3. Panther Grouper ( Start small and you can watch him gow like a weed!)
4. Queen Angelfish( Not really aggresive but beautiful none the less.)
5.Spotted sweetlips( acts like a dogs wants you get it established)
6.Princess parrot( very shagadelic)
Thats all I got for you cornshuker