FF6 and FF7 are good and simplistic RPGs that are easy to learn. They both have solid stories as well. The major difference I find is that FF7 has higher heights, but also lower lows, whereas FF6 is fairly consistent throughout. I also like how the World of Ruin in FF6 is open up while the World of Balance is a straight narrative.
FF7 starts off VERY good with the whole Midgar section, terrorist eco-action against Shinra, then travelling around the slums picking out women's clothing; then dressing up in drag to get into a perverted Mafia leader's mansion, and none of them can tell that Cloud is not a woman (kind of reminds me of the movie Boondock Saints); FF6 was not that brave. Unfortunately, once the player is past Costa Del Sol (the beach resort town, which reminded me of Earthbound's beach resort town) then the game lulls for quite a while as they chase Sephiroth around the world; FF6 also doesn't have any parts that uninteresting.
My preference goes to FF8 though; largely because the towns are more diverse than any other FF game, and there is a lot to do in them. Plus the towns and people evolve as the game moves forward, which makes it worth going back and visiting people multiple times. The Garden in FF8 is the most interesting, since it goes through many sections of the story, and the people respond to that - they also evolve as the fame of Squall and company increases, they go from ignoring you, to knowing who you are, to looking up to you as someone who has accomplished great things. The towns in 8 are as diverse as a archology/University, a pastoral village, a 1920's Paris, a WW2 era Berlin, a Futuristic Metropolis, a beautiful port city with a resort, an eco-paradise underground city, etc... It has a lot of places I would love to visit.
FF8's system is much more complex than FF6 or FF7. Wherein FF6 and 7 you buy stuff and equip it, and you level up just by killing enemies - FF8 opens it up much more, you can kill enemies and refine the dropped items into magic, or if you are lazy you can outright draw magic (but this is incredibly tedious and I don't recommend it), or you can draw magic from draw points, buy items and refine them into magic, or play cards and refine them into items and magic (this can be the most powerful and quickest method if you are good at the card game); you equip this magic as you would weapons and armour in previous FF games, except with many more options that directly influence each stat rather than pieces of armour which effect multiple stats, etc... Players can equip any number of summon creatures, they gain AP in battle and learn skills (each creature has something like 50 skills to learn), you can customize them with scrolls as well that can be acquired through the game (mostly in the late game); when skills are learned, they are either automatic (they'll effect the player regardless) or equipable (player must equip the skill in order to have it). You do not have to worry about which characters you level up as power is linked to what you have equipped; this can be transfered from character to character in a single command, or if the player wishes, can partially transfer any amount of magic/skills. It is much more complex, so this game will probably put off inexperienced players (I see new players who tink the only way to get magic is by drawing from monsters); but I think players who know the system well will like this game best.
Plotline and characters, some people don't like them; they say Squall is emo; but Cloud is actually the one who most fits the emo persona with his emotional-hardcore punk look; but they forgive this; I think this is just an excuse to hate a game whose gameplay they didn't grasp while other people seemed to be able to grasp it more easily. I personally love the multi-threaded approach to FF8's story, how in the end it all comes around to the beginning of the sorceress again. It is the only plot in the FF series that has any real amount of complexity to it. Xenogears is probably the only game I have played which has a story in it that I liked more, and felt had more complexity.
In other words, I loved FF6 and I loved FF7, but FF8 is the game that leaves me most satisfied in the series.