Season One: Murder House: A family move into their new house, a house that becomes intent on making their life a living hell. This house makes both versions of the Amityville Horror look like a kiddies movie. Murder House is amazing and stands out as the best in the series.
With the second season, credit due to the producers for not trying to do more of the same, but in Season 2: Asylum, we are given a bat s**t crazy story that pushes the comfort zone on a lot of fronts. How they ever connected all the sub-plots plots in this season I will never know. Asylum is okay viewing, but it will make you miss the simple storytelling of Murder House which wins you over with its sinister revelations rather than nasty gore fest.
Season 3: Coven, brings us back to a more subtle form of story telling...well at first, but like Asylum the story does have some truly horrific parts to it. If you think Kathy Bates was bad in Misery, here she is evil incarnate and she's not even one of the witches. Her special power is malice and cruelty and these things she does so well that she gets invited to season 4.
Season 4: Freak Show has to be the poorest outing, though there were ample chances to maintain the standard, this season never really takes off and the best characters seem to appear for the shortest time. The story doesn't really grab you or have you rooting for a main protagonist and so I hope in Season 5 they will combine the best of seasons 1 and 3.
American Horror Story has grown so much in popularity that it comes as no surprise that the show has not just been commissioned for a fifth outing, but has also spawned a spin-off show entitled American Crime Show. It has been rumoured that he first season will be based around OJ Simpson murder case and trial.