Out of the ones you mentioned I primarily suggest Final Fantasy VI. It's my third favorite game in the series, and all around a pretty darn good game. Pretty memorable music and a great villain, although bit old fashioned.
Final Fantasy III is another one of my favorites, it's simple yet addictive and engaging. It's challenging for sure, though in a way you have to grind more than in other games, in other words nothing serious.
As someone who does not think highly of Final Fantasy IV I found The After Years to be surprisingly good. It can get somewhat tiresome near the end thanks to huge amount of partially recycled boss battles.
I found Final Fantasy V to be by far the worst one in the main series. It has a great gameplay, though it fails at everything else. Who knows, you may actually enjoy it far more than I did.
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