I'll just take only normal difficulties in account, since that's what the devs balanced the game for (which is to say, some game systems are completely broken by easier/harder difficulties, be that in your favor or against your favor), plus some games just don't have a difficulty option and i felt it unfair to not consider them just because another game has "story difficulty" where you can't game over unless you *really* try.

Easiest: This one was easy to get down to two contestants: Last Rebellion and Rhapsody: A Musical Adventure. I'm giving it to Last Rebellion because on top of being (arguably) the easiest RPG I've played, it was also definitely the easiest platinum trophy I ever got.

Hardest: A lot tougher to narrow this down, I would say probably Super Robot Wars Alpha Gaiden. SMT Nocturne is pretty high up there (I haven't played enough of Strange Journey to judge that one), but the thing with SMT is that you can always grind and improve your party setup if you get stuck on a boss. SRW Alpha Gaiden is one of those SRPGs that are purely linear and you can't grind in them, so when in the last 5-10 stages it became clear that my team just wasn't up to beating the bosses (one with a time limit, even), my best choice was to actually completely restart the game.

That said, my hardest pick is purely subjective because I'm just not good with having to manually build my teams with their stats and skills. Much better at on-the-fly decision making and reacting, which is why action RPGs were never even a consideration for my hardest pick, and both my contenders are heavily about building your team.