i've played EVERY SINGLE final fantasy, gameboy, mystic quest, japan/english u name it i've played at least some part of it....
except for final fantasy 9

but i loved every single one...
my favorite would be between 5 and 6, just because of the new systems they introduced and how they captivated people...for weeks...
4 was great...i mean you had 3 worlds!!! worked great...the dancing calbrenas still creep me out
7 was awesome...definately the best rpg of the polygon era...
8 wasn't up to SQUARESOFT standards...but it would still crush a whole lot of other RPGs...
i think everyone would have loved it (or liked it more) if it wasn't called "final fantasy 8" and had another title...just not worthy of the Final Fantasy name, i guess...
10 is great, i think...much more linear path and no real "freedom" was kinda awkward, but it does have a tight rope to walk story-wise....graphics blow me away everytime too.
4,5,6,7 stood out the most to me because it made you want to keep playing, keep going...
sidegames/quests::
6- battle arena - you can bet your priceless atma weapon, earn an elixer!!! (or something like that)
7- chocobo breeding, all of gold saucer - omg those chocobos pissed me off...but so worth it when you see that shiny gold one! and gold saucer!!! like an arcade!!!
8- cards - great strategy, even better pictures on the cards
10 - blitzball - too easy, not interactive, too long, but sometimes i just play blitzball for about an hour and save and quit...