Can't believe no one has asked this question before!!!
For me I think the best is Season One which starts brilliantly with 'The Fourth Horseman' and then the ramifications of the death outbreak ripple on through the remaining episodes of this run.
my next favourite would be Season Three which gets our heroes back on the road, grime and all.
Least favourite, despite having a number of standout episodes ("Greater Love", "The Lights of London", "The Chosen" and "Paracites" to name but a few) it has too much of cosy business as usual 'soap opera' feel to it.

