Mmm, I don't know really. Final Fantasy 4 was easier in the end because of the inclusion of save points in the game,and generally the series has become easier each outing, where as in Final Fantasy 10 (I think it was 10, but could have been one or two before) when you touched a save point you were completely healed. That took a lot of the challenge out of the series right there, no worrying anymore if you could make it to the end or out without dieing or running out of MP or healing items.
Final Fantasy 3 has always been known as like the hardest game in the series. The ending to Final Fantasy 5 can be a bitch, (I don't know about the GBA version because I haven't had a chance to play that yet.) but the final area monsters don't give any experience so you fight the final boss as strong as you came in.