What big fish for a 90 is too much of a broad question, how big do you consider to be big, what kind of fish do you like, is this to be a single specimin kept alone or do you want to be able to mix tankmates, are you looking for something that shows personality and watches you through the glass or an oddball that looks unique, do you want it to have bright showy colours or are you happy with a fish that is brown but has a huge mouth?
As Tokis says the dimensions are importent too, i have a few 80g tanks that have the same footprint as a 120 but are only 18" high as opposed to 24", but a tank of the same volume but taller and thinner would not be able to support the same sized fish due to a smaller footprint.
Also what is available to you and how much money do you want to spend, we could name an endless ammount of fish but if you cant obtain them or they are more expensive than you would be willing to pay then it is pretty pointless.
You need to have a think about what fish you hav seen and like and then draw up a shortlist to check if they are suitable or not.