Tough question
Best movie? Probably. BUT would ANY OF THEM even exist if Iron Man hadn't taken off into the stratosphere under Favreau's stellar direction and extraordinary attention to detail? If the casting of RDJ hadn't been so spot-on perfect that he actually inhabited the character? Because if Iron Man had collapsed like a pancake, we might never have made it to The Avengers.
So yeah, I agree it's my favorite of the bunch. But I have to qualify that by acknowledging that the success of Iron Man may very well be what made all of it possible still it's just an opinion.
"You told me I had nothing. But you were wrong. I have love, I have hope and I have faith."

Yeah I'd say so. Though Civil War and Guardians are pretty high up there too.

Still the best CBM period.
Some great contenders but this is still the champion.

Yes but the margin is shortening.
I don't seek justice. I stalk it!- Misty Knight

I like Spider-Man 2 way more than this. The story in it felt more personal, the stakes were higher, and action was definitely way better.

Nope! I must have missed that! I still haven't watched any of these MCU movies from beginning to end! I see bits and pieces; attention span the size of a gnat due to sleep apnea problem! Even though I'm more into DC universe with Batman and Superman, I still haven't seen all of their movies entirely; esp.
"TDK"
&
"TDKR"
which I thought were much too long! Still waiting for the last 2 offerings to be telecast on cable; even
"Batman v Superman: Dawn of Justice!"

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• • Sci-fi, Batman, & E:FC

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• • Homage to DW & B7

IM wasn't that far up. Remember in AOU how he says, "300 feet above us." So, from the top of the building add 300 feet.
They also weren't expecting him to come back at all, and not falling like a rag-doll.
Thor also is that fast. Remember when he took off from Clint's house? He zipped up faster than IM usually does.

IM was almost ready to hit the ground and Thor was still waving around his hammer! He's not that fast; except maybe in the animated series!

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• • Sci-fi, Batman, & E:FC

http://www.childrenofrassilon.com

• • Homage to DW & B7

esp. since the rest like THOR just stood there watching!
Actually, Thor was winding up to go catch him when he saw Hulk go after him.

It's right up there even though Batman and Superman are more iconic! I still get verklempt with Tony's sacrifice going into that hole with the missile! The Hulk catching him as he falls back to Earth is very moving to me; esp. since the rest like THOR just stood there watching! Boy, he would have been late! The animated series dealing with Ultron being destroyed in the son was also quite memorable! Thor went out to bring Tony back and both had to be caught by the Hulk before crashing back to Earth!

http://scifiblogs3.blogspot.com/

• • Sci-fi, Batman, & E:FC

http://www.childrenofrassilon.com

• • Homage to DW & B7

Best Mcu movie, definitely.