I picked Season 1.
FatBurger is right, it was tempting to choose Season 4, especially since we've finally started receiving so many answers.
But. I don't think any season has completely blown me away in as many ways as Season 1. Seeing for the first time that Locke was in a wheelchair, seeing the light come on in that hatch, the mystery of Danielle and the Others, before we really knew even who the Others were. All those cross-overs in the back stories, where we'd see Hurley giving a press conference after winning the lottery on the television in the house where Jin was beating a guy up for Mr. Paik, or Sawyer as a customer in Kate's mother's diner.
So many pants-blown-off moments, so many times when that crooked LOST logo came floating toward me on the black screen to mark the end of the show and I literally screamed, "No!" -- I don't think the emotional aspects of those moments have been as effective as they were in Season 1.
Now, we've kinda gotten to know the minds of Cuseloff, we know to expect big surprises, we can speculate more informedly and sort of have an idea of where things are going. Everything was new and fresh and unknown in Season 1.