It's pretty hard for me to choose and made it very hard to advance in ff14 because I always wanted to be something else.
To me Ninjas, Mystic Knights, monks, regular Knights, white mages and Dragoons are always classes I want around.
FFT made me hate a lot of Mage classes because of the long charge time. A ninja always did the job better in my experience.
Mystic Knights are very underrepresented. IIRC Celes qualifies as one along with maybe a few FFT guests and the combined powers of Vivi and Steiner. Love that they borrowed Minwu's dope design.
Monks are always around in the series until FF10. They are so good early game and have had great representation with Joseph, Yang, Sabin, Tifa, Zell, and Amarant.
Ninjas also have been around and are usually the best class in the game and have some cool characters like Edge, Shadow and Yuffie. We need another ninja main character.
Depending on the game Dragoons are pretty good. Never used them much in FFV. I like the characters that have the dragoon role.
Regular ass Knights and white mages are perhaps nothing fancy, but always reliable to do their job. What main characters are considered a pure knight is a little blurry becaus they will sometimes take elements from other subtypes of Knights to be more interesting. Steiner sticks out as being just a good Knight ass knight. White mages are a little more obvious. While had the white magic spells and some sort of staff or long range weapon. Typically the love interest.
I think if I picked up ff14 again I'd go with monk over my initial choice of dragoon. There are just so many badass monks in the series. Hoping we can return to a day where the ff games have characters with customizable yet defined jobs like FF10, 9 or 6. I want to look at the next black Mage and say "hey that's the black Mage" even if it's not wearing the hat. Sort of like Lulu.