In my opinion, there are basically two standout movies from the various phases of the Marvel Cinematic Universe (MCU) so far: the first Captain America (Captain America: The First Avenger) and the first Guardians of the Galaxy. You can watch both as standalone (and in any case Guardians of the Galaxy doesn’t really fit with the Avengers movies anyway). The first Iron Man also has its merits, but suffers from some pretty extensive misogyny. If you watch any Avengers at all, watch Age of Ultron.
In fairness, these movies are a complicated combination of competing factors. I am more interested in the interpersonal dynamics, but for legitimate reasons of fan interest, the movies pretty much have to contain aspects of fan debate and set-piece battles.
The fan debate items are things like:
“What happens if Thor’s hammer Mjölnir hits Captain America’s vibranium shield? (Answer: A sort of giant shockwave) or
“Can The Hulk lift Thor’s hammer Mjölnir?” (Answer: no)
Unfortunately one of the main fan debate items, and one which is also cinematically cost-effective, is What happens if character X fights character Y? Which means we end up with a lot of engineered situations of Avenger fighting Avenger.
The set piece battles are similarly popular with fans, but end up basically just being a jumble of people doing super power things until the bad guys inevitably lose.
The MCU is currently broken up into three phases. They have various Avengers, plus Spider-man, but not the X-Men (due to obscure studio licensing issues). The X-Men is a completely separate universe cinematically.
Iron Man (2008) – Iron Man 1 – watch
The Incredible Hulk (2008) – I didn’t view it
Iron Man 2 (2010) – skip
Thor (2011) – skip
Captain America: The First Avenger (2011) – Captain America 1 – watch
Marvel’s The Avengers (2012) – Avengers 1 – watch
Iron Man 3 (2013) – watch but skip the set-piece battle
Thor: The Dark World (2013) – Thor 2 – I didn’t view it
Captain America: The Winter Soldier (2014) – Captain America 2 – watch
[ Guardians of the Galaxy (2014) – Guardians of the Galaxy 1 – watch but it really doesn’t fit with the rest of the Avengers movies; you can watch it standalone ]
Avengers: Age of Ultron (2015) – Avengers 2 – watch
Ant-man (2015) – skip
Captain America: Civil War (2016) – Captain America 3 – watch but skip the battle at the end
Doctor Strange (2016) – I didn’t view it
[ Guardians of the Galaxy Vol. 2 (2017) – skip – long sequence in the middle where they are captured on and then escape from the ship is cringe-worthy and celebrates gratuitous violence ]
Spider-man: Homecoming (2017) – Spider-man reboot 3 movie 1 – I haven’t seen
There are a few sequences you could watch, for example Iron Man 1 & 3, or Captain America 1, 2, 3.
There are lots more movies lined up. You can see the current plans at e.g. https://marvel.com/movies/all
They are basically positioning everyone for the Avengers Infinity War movies (the battle for the Infinity Stones) in 2018 and 2019. That will close a 10 year cycle of movies, I’m not sure where they will go after that.