I'd have to say VIII gets the most unreasonable criticism. Sure I'm pretty outspoken about some of its aspects - most particularly, it's horrible neglect of the secondary characters, especially Quistis. However, despite all the issues people have with the gameplay, storytelling and characterisations, FFVIII utterly captivated me from the moment I first played it. I was totally enthralled the whole way through (except for a slow period near the start of disc one), and the ending had me on the edge of my seat until that final post-credits clip.
Granted, I first played the game when I was about 16, so my outlook's a bit different now, but it's still a mighty fine piece of work. It had a different combat system and overall style, which was nice. A lack of new ideas is the worst thing that could happen to the series, so it was good that FFVIII experimented a little.
FFX-2 gets a whole lot of unreasoned criticism, too. For some reason, people expect the FFX sequel to have a whole new world with new characters, all-new locations and a completely new plot. Rather daft, seeing as it's a sequel set in the same world. Granted, the non-linear style makes it harder to appreciate the overall story; you have to try in order to appreciate its depth, which means you have to be intent on enjoying the story if you want to go through the trouble. The style's too upbeat, people say. Again, it's usually just complaint because they've been faced with something new. Characters who begin their journey having fun and living life, rather than facing a crisis from the outset. Too much like Charlie's Angels? I find this rather daft. It has three women who travel the world and dispatch villains while having fun, but that's as far as the similarity goes. Might as well call FFVII a Charlie's Angels clone, as it meets the same criteria.
On its own, X-2 doesn't live up to the standard set by the other games. But then, I don't feel it was ever meant to. It's an extra chapter to the FFX story; a coda about Yuna and the future of Spira. They could've made another vast adventure as big as the original FFX, exploring all the characters and their journeys, but there would've been a real risk of that feeling tedious and hopelessly contrived after a while.
Out of these two, I'd still say that FFVIII receives the most unreasonable criticism. X-2, despite what I consider its strengths, has plenty of grounds for criticism, especially in contrast to the other games in the FF series.