I found the #2 like wtf that is one of the press the "I win" button moments that ruin ME3 and they makes it sound like the save them both choice is hard to obtain it is almost 100% guaranteed if you make rational choices over both ME2 and ME3.
Number #1 was one of the worse moments in the franchise for me because it was the first time we have evidence that the writers had created the cost free victory outcome. That they showed they were afraid to create an outcome that at its best wasn't unicorns and rainbows.
Mordin's death is a great example of how to write in a scripted death and Virmire is a great example how to write a choice with real consequences, death. Where there are no paths to a "perfect" outcome.
Meeting sovereign and vigil are both in the top three for me I'd almost put them as a tie for number 1. I go back and forth as to which was the most impactful.
But these are subjective and what is impactful to me isn't impactful to others and I am guessing that if you didn't play the games as a series the impacts will be different. What i mean by series is when I ply ME i always start with the previous game then play the current one. So when Me2 came out i played ME1 first and when i finished I started me2. Now I obviously start playing the earlier games before the current game is released so I am ready on release day but my playthrough is of the series not the current game with a save from the previous game that I haven't played in years or months. Obviously when ME3 came out i started with Me1, then immediately jumped to Me2 then started me3. It was actually playing the series like this that convinced me the ME2 while the most liked of the series is the weakest written of the series. You can removed Shepard from the story and put in any Human commander and poof nothing ruins the story. You can't remove Shepard from Me3 or Me1 because Shepard is the first spectre so is required in me1 and me3 is when all of Shepard's warnings come true. Yet Me2 really doesn't need Shepard, Jacob or Miranda could have been in command and it would have worked fine story wise.