Every series is amazing! Series one is good, series two is great, series three is almost perfect and series four is amazing! I rank the series like this:
1.) Children of Earth 2.) Miracle Day 3.) Series Two 4.) Series One
I can honestly say that they get better as they go along! The only problem with Miracle Day is that the episodes drag on slightly and every two episodes should just be one. Children of Earth was a perfect length!
Definitely buy this if you have got any of them yet! Completely worth it!
(Make another series before John Barrowman gets too old to play the immortal man Russell!)