Out of what I've played, 9 > 8 > 10 > 7.
FE9 has the my favourite characters and storyline by far. FE9 is my second favourite game.
FE8 has the most replayability and its plot isn't as bad as people make it out to be. I love most of FE8's characters too, but not as much as FE9's.
FE10 has excellent gameplay, but it has no character development, an incredibly flawed plot and MICAIAH.
FE7's overrated. It's full of plot holes, has mediocre characters (except Pent) and has the worst gameplay out of the ones I have played.
Overall, (FE13 >) FE9 > FE5 > FE8 > FE4 > FE10 > FE7 > the rest, since I know little about them.
FE5 has wonderful gameplay and my favourite protagonist. It doesn't have much in terms of characterisation, but I can excuse it.
FE4's pretty overrated, but it's still a fantastic game with alright characterisation, a wonderful story and SETY AND LEAF (like FE5).
FE13 hasn't been released outside Japan yet, but looks to be the best by far.