I played Genius and DoD. I don't remember playing Rebellion. I probably should.
Anyway my lack of knowledge/memory of NoR aside, Genius and DoD aren't dependent on each other story-wise, so whichever you try first is up to you. Genius came out first, so I played that first, and loved it, but DoD was easier, though still more difficult than your standard turn-based traditional style JRPG. If you ever get dissapointed in, or frusterated in a game due to difficulty, DoD is probably the better starting game out of the two.
Another thing about Genius is that it is pretty tough late game, and oddly enough, grinding more and more doesn't actually help you THAT much. So if you're stuck, and you are more likely to grind to win rather than change your strategies, it can kind of brick wall you like that. It still DOES help, particularly with the super rare end game crafting materials, but you don't need them to win, and they are kinda stupid rare to the point where if you grind for just a few, it will take you like a dozen hours+. Though you can try and speed up the process with cheat engine or something if you aren't against that.
That being said, Finally defeating the final boss of Genius was really something. god that boss was brutal, but when I finally beat it, no other turn-based rpg boss ever made me feel like I really accomplished something like that one. It was like 1ccing lunatic eosd for the first time.
The games had other differences, not just difficulty, but honestly I think that's the important one. I can't imagine someone disliking one and liking the other for any reason other than that.
