In my 90g our two centerpiece fish are a German Blue Ram and an Angelfish. They are awesome!!!
The most common four fish that are thrown out are Rams (German Blue, Electric Blue, Bolivian), Angelfish, Gouramis (dwarf or otherwise), and Bettas
Betta - actually makes a nice community fish, but you can't have other fish that are "finny" that the Betta might mistake as another male. You cannot have two bettas together, either male or female.
Angelfish - Ideal as a singleton, but CAN be kept together if you happen to get two with a nice temperament, or buy an already bonded pair.
Rams - Similar to Angelfish, better as a single or a bonded pair, but you might be able to fit two different types in your 90
Gourami - best suited alone
Gouramis, Bettas and Angels don't really get along and tend toward the same area of the water column so it's best to pick only one of these. You can have a ram or two with any of the other three as rams usually hang near the bottom.
You can always get lucky and have fish that end up getting along like an Angel and a Betta, or Betta and Gourami, etc etc but this is totally up to luck so most people don't risk it.
Disclaimer: fish profiles aren't my specialty, so I may be corrected on some points