-FF Origins: A bundle of FF1 and FF2. This version of FF1 is the closest you'll get to the difficulty of the NES one, but with some nice things like running and auto-target. FF1 lets you choose 4 guys and make them whatever class you want from 8 choices. It's probably the most open FF game since it rarely tells you where to go.
FF2 is considered one of the worst FF games, though I kinda enjoy it. It's more story-based, giving you a bunch of pre-named characters with portraits and all that. You level up by specific stats instead of levels. So if you use swords a lot, you'll get better damage with swords, if you get hit a lot your defense will go up, etc. It's one of those love it or hate it kind of games.
-FF3 is one of my favorites since it
has Onion Knight has a lot of new classes and a lot of places to see. It's also probably the hardest FF game, so get this if you want a good challenge and a lot of freedom with your classes. The PSP version that's available has 3D models for characters/enemies, and comes with the original NES soundtrack as a replacement for the arranged version that's usually in-game.
-FF5 is my favorite. Partly because its job system is even more fleshed out than FF3's, but also because the story never takes itself too seriously. And the main character's name is Butz. Also the main villain is a tree. Yeah.
Anyway, there are more classes in FF5, and unlike the static jobs in FF3, you can mix abilities between classes. For example, if you level up Ninja enough, you can learn to dual wield, which you can carry over to say Dragoon, allowing you to dual wield spears. Combining the right abilities can even create incredibly broken set ups. It offers a lot of variety, so if you like the sound of mastering each job and experimenting with the combinations then this is what you want.
-FF6 is heavily story-driven, with set characters that have set jobs. It's also considered the best in the series by quite a few people. I haven't played enough to really talk about it too much, but it's more or less linear until pretty late in the game. So if you want a really good story and characters then there you go.
-Dissidia Duodecim is the better version of the original Dissidia that I talk about before. It's FF Smash Bros. with a lot of fanservice (sprites and references, not tits). You can even play as the main villains and GIRUGAMESH.
So yeah, hopefully that's enough info. I can always clarify something if needed, just let me know.