Having played through all of them, EOIII and IV are a much better start- they are far more forgiving in general, have fewer floors per dungeon, and also the subclassing system lets you cover holes in your party abilities, particularly healing, much more easily. Plus the boat/balloon exploration helps with the pacing, giving a nice break from the main task of dungeon crawling.
EOU has an option where you can play though with a set party, with cutscenes, or your own setup, and the story it uses (EO1) is pretty good. But all of them are 50hr+ games- I'd go with the one you can enjoy the mechanics of in the long term, and for me the increased party depth and skills that feed into each other over the classes and subclasses of the later games makes them much more interesting to tinker with along the way.