That tank gives you a huge amount of options.. It all depends on what you're interested in. Do you like personality? Color? Aggression? 1-2 big fish? a number of small fish?
you could look into vieja/parathraps. Also, some of the parachromis species. I recently ordered myself some Red Tiger Motaguense, and Parachromis 'La Ceiba'.
Like was previously stated, you have lots of options. For more personality I've had many people tell me fish from the Amphilophus group can't be beat (haven't had one yet myself). Also as stated the SMALLER parachromis is something you might enjoy (dovii is too big for a 90 gallon full grown).
Never had a syn but others on the site will hopefully chime in. Imo there colors cannot be beat simply beautiful fish. Not sure how personable they are as a wet pet though. Argentea are gorgeous and nasty as hell. But may not have the personality your looking for the trade off is worth it though
Edit: cant knock americancichlids suggestions either freddy's and loisellei are freakin awesome looking would love to own one down the road.