This strikes me as a really unfair question because only two of the seasons are directly comparable.
Season 1 is a normal season, 13 episodes is a bit long for traditional UK series but I think they're starting to lengthen them to make it easier to sell to the US. Season 1 is very much a show trying to find its feet (I am being nice).
It's uneven. It's clunky. Some of the episodes are howlers. On the other hand, some of them are very good and set up some later stuff.
Season 2, again 13 episodes, is a show that knows what it is doing, mostly. Except for that one section of "Fragments" that everyone agrees is non-canonical. It also contains the "Day in the Death" three-parter and "Day in the Death" itself is one of the better 45 minutes of TV I have seen. Season 2 still has its problems (Grey in particular could do with better writing and a different actor) but it's solid.
In Season 3 (or Children of Earth), the format changes completely. It's 5 one hour episodes that centre on one story.
Children of Earth was fantastic. The remaining Torchwood characters were solid, well-acted and written, the new characters were amazing. Peter Capaldi's Frobisher gets all the plaudits but Susan Brown's Miss Spears is even better. The pacing, and the ratcheting tension are brilliantly well done, leaving more time for one story means it can be covered in greater depth.
Season 4 (Miracle Day) loses a lot of that. Part of the problem is the increase in length. Large parts of the first section felt like it was padding, treading water waiting for the real plot to begin.
Secondly, it didn't feel real. I think some of it was shot in the US, but it felt like they'd shot it in the BBC's best mock-up of California. I didn't believe in their version of the world. In expanding the focus, they lost something. (Also, as
selenak said [severe paraphrasing ahoy] there's probably one country on Earth that might have a problem with sending its citizens into gas chambers.)
Thirdly, and most importantly, I didn't like most of the new characters. Even an actor of Mekhi Phifer's enormous charm (and ability) can't make Rex feel more than an Owen retread. I stopped watching when the thing happened to Vera Juarez, because she was the only one of the new people that I even liked.
So yes, either season 2 or Miracle Day but they're so different that it's like comparing apples and oranges.
Day 1 -
Your favourite Torchwood member Day 03 - Your favourite episode
Day 04 - Your favourite alien
Day 05 - A scene that made you cry
Day 06 - A scene that made you smile
Day 07 - Your favourite quote
Day 08 - Your favourite cast member
Day 09 - An image that makes you smile like a git
Day 10 - Your favourite promo picture
Day 11 - Your favourite coup
Day 12 - Your favourite couple scene
Day 13 - A piece of music you love from the series
Day 14 - Something you’d like to re-write in the series
Day 15 - Behind the scene video that made you laugh the most
Day 16 - Your favourite outfit
Day 17 - Your favourite Gwen moment
Day 18 - Your favourite Ianto moment
Day 19 - Your favourite Jack moment
Day 20 - Your favourite Owen moment
Day 21 - Your favourite Toshiko moment
Day 22 - Who you’d shag, cliff and marry
Day 23 - Your favourite Torchwood website
Day 24 - A Torchwood geek moment you’ve had
Day 25 - Your favourite Torchwood fan-fiction
Day 26 - A piece of fan-art
Day 27 - A fan-video
Day 28 - Your favourite location or set
Day 29 - Where you think the series should continue to
Day 30 - Best Torchwood on Doctor Who moment