[Buzz201]

I think the definitive order is theatrical/television release date, take note that Marvel's Agents of S.H.I.E.L.D. ties into Thor: The Dark World and Captain America: The Winter Soldier, so you'll probably want to watch those in the middle of MAoS...

So, I believe it works out as:

01. Iron Man (2008)
02. The Incredible Hulk (2008)
03. Iron Man 2 (2010)
04. Thor (2011)
05. Captain America: The First Avenger (2011)
06. Marvel's The Avengers/Marvel Avengers Assemble (2012)
07. Iron Man 3 (2013)
08. Marvel's Agents of S.H.I.E.L.D. Season 1 Episodes 1-7 (2013)
09. Thor: The Dark World (2013)
10. Marvel's Agents of S.H.I.E.L.D. Season 1 Episodes 8-15 (2013-4)
11. Captain America: The Winter Soldier (2014)
12. Marvel's Agents of S.H.I.E.L.D. Season 1 Episodes 16-22 (2014)
13. Guardians of the Galaxy (2014)

But it should still make sense if you watch them in any order (perhaps with the exception of Agents of S.H.I.E.L.D.)...