Battle Moon Wars (PC) is a great strategy RPG, but not available in English...
Lufia and the Fortress of Doom has a rather steep leveling curve, the sequel (Lufia II: Rise of the Sinistrals) is considerably easier and more forgiving.
Super Mario RPG can be easy unless you do the "no equipment challenge" some people do, in which it can be an amazingly difficult game to get through unless you use certain characters for certain fights (Yaridovich and past is near-impossible without Geno Boost...)
I personally loved Saga Frontier, and it comes with a battle setup that prevents you from hitting a certain level of strength (because once you do, the game levels up monsters as well, as a means of saying "You've had enough, move along now") - everyone seems to either love or hate this game, not too sure what you'd think, but it has a very long playtime as well as good difficulty. I HIGHLY, HIGHLY, HIGHLY recommend never, ever, ever touching any of it's sequels, however.
The Shining Force series (the first chapter has been rereleased for GBA) is a good strategy game with some nifty features and hiddens characters, but it the remake comes with some corny SFX/BGM as well as some "character adjustments" that leave many of the best characters (Khris, Xylo) rather useless in comparison to how good they were in the Sega Genesis version. Personal favorite of mine, though I'm unsure which version you'd like more - the GBA version has greater difficulty but many characters are nerfed so badly that you may as well just send the main character in to do all the work. In the Sega version, the hero was actually one of the weaker characters, whereas in the GBA one he's by far the strongest.