I just switch around TIH and IM2, since the Stark scene in TIH is meant to take place after IM2. Otherwise, release order. The main story of Cap:TFA is meant to be a flashback, not the "present" of the film.
Agreed. TIH ends with Tony Stark already acting as a consultant with SHIELD, which doesn't officially happen until after the events of IM2, so it makes more sense to watch the latter movie first. IM2, Thor, and the bulk of TIH all take place around roughly the same time, covering about a week in-universe, while the rest of the MCU pretty much continues in real time.
Guardians of the Galaxy's cold open was set in 1988 and then when it jumped to the present it explicitly said "twenty-six years later," so it took place in 2014, the same year the movie came out.
This is generally how I order the movies:
1. Iron Man
2. Iron Man 2
3. Thor (because Coulson's departure in IM2 leads directly into this movie)
4. The Incredible Hulk
5. Captain America: The First Avenger
6. The Avengers
7. Iron Man 3 (Christmas 2012; dialogue places it six months after The Avengers)
8. Thor: The Dark World (2013; dialogue indicates two years have passed on Earth since the first movie, which came out in 2011)
9. Captain America: The Winter Soldier
10. Guardians of the Galaxy
11. Avengers: Age of Ultron
12. Ant-Man
13. Captain America: Civil War
And if you want to include the Marvel One-Shots...
1. Iron Man
2. Iron Man 2
3. A Funny Thing Happened on the Way to Thor's Hammer...
4. Thor
5. The Incredible Hulk
6. The Consultant
7. Captain America: The First Avenger
8. Agent Carter
9. The Avengers
10. Item 47
11. Iron Man 3
12. Thor: The Dark World
13. Captain America: The Winter Soldier
14. Guardians of the Galaxy
15. All Hail the King (not explicitly dated, but I put it here to give a decent amount of time between it and IM3)
16. Avengers: Age of Ultron
17. Ant-Man
18. Captain America: Civil War