To me, series 3 and 4 are the most consistent in terms of characterization and writing, although 1 and 2 definitely have standout episodes. I think if I checked my play count on iTunes, 3 would have the most replays.

If I had to rank it would probably be 3, 4, 2, 1, with 2 edging out 1 slightly because although H and K are two of my least favorite episodes, I, J, and L are three of my favorites.

I sort of wish Birling day had been a one-off plot device but that's neither here nor there.