I think season 1 will always be my favorite because it held so much promise and we got to know the most important information about each of the main characters who hung around for the whole show. Everything was new- people, the Island, the music. So much was still open to interpretation without stalling the progress too much. The further into the series we got, the more defined things became, leaving less wonder and more 'um...okay.'
Without the luxury of re-watching pretty much any episode in six years, I'll go with 1>4>5>2>6>3
