Part 1: Resistance
Honestly the easiest mission out of the three currently available at the time of writing this. And a great introduction to the missions even though we get introduced to a mechanic that never gets reused in later missions. Although we do get one of the only two playable appearances of one of the best heroes; Lúcio.
Part 2: Liberation
The hardest mission out of the three with those stupid run ending Subjegators. Most of my runs in Liberation were ended do to this stupid piles of actual dog shit. What this mission can be summed up as hoard battles. Also in the middle of the mission after retrieving Reggie is the most bullshit part of the entire mission as you have to keep tabs on TWO OMNICS while taking out the entirety of Null Sector. My favourite part is Claire wanting a gun but your support declining it. Also Reggie has some of the best animations in the entire mission.
Part 3: Iornclad
My favourite mission out of the three currently available. We get introduced to the best enemy in Invasion, the Titan. Not only being completely invincible to all normal attacks but gets the best death I’ve seen. Though it gets annoying at times, but it has its pros as well.
Bonus: Underworld
Although only being an event mission back in season 6, it’s still technically apart of Invasion. It’s short and sweet, and gives us a good introduction to the Invasion missions. Though no longer playing as of now, I’m hoping that it becomes playable during Overwatch 2’s anniversary (I know this is Blizzard we’re talking about, and it’s probably not gonna happen).