I might as well way in on this.
First, I'm going to say it outright. Mass Effect 2 is my favorite game of all time, much less my favorite in the series. Now let me explain why:
I agree with you all that ME1 has the best nostalgia to it. It was a fantastic game that first introduced us to the universe and made us care about it. I'm also going to say that it has the best story. The races, worlds, people, and Reapers are set up so perfectly in this game. Soverign is fantastic, Saren is twisted, and the main plot has that "godly air" to it, and to the mystery of the Reapers, that I think is taken from us in ME3. ME1 was the perfect setting up game. It made me want to play Mass Effect 2. But I think ME2 does much more.
ME2 does, yes, probablt have the weakest main plot to the series. But that's not what matters in ME2. What matters is the characters. It is easy to agree, at least I hope, that ME2 has the best character plots/interaction of the entire series. ME1 made me care about the the universe, but ME2 made me actually care about who is in it. It takes the characters, both old firends and new allies, and even new enemies, and puts you right up next to them and forces you to understand them. Every loyalty mission, every side converstaion, every meeting with the Illusive Man (major points coming from TIM, Martin Sheen was excellent) makes you care more and more about these people you're going to fight through hell with, and through that, you care infinately more about the universe.
Let's put it in terms of ME3:
I cared about curing the Genophage because I cared about Wrex, Mordin, and Grunt.
I cared about making peace on Rannoch because I cared about Tali and Legion.
I cared about saving Palaven because I cared about Garrus.
I cared about Thessia because I cared about Liara.
I cared about Earth because I cared about Shepard, Anderson, and all the humans.
I hated Kai Leng because he killed Thane.
And I hate having to pick Destroy because I cared about Legion, EDI, and Joker.
(Here's a good example: How easy would it have been to pick Destroy if it had taken place at the end of ME1, when you hated the Geth and EDI was a nonfactor?)
And I'm not saying that I don't care about anyone BUT these people in the universe, but I see the Krogans, Turians, Quarians, Salarians, Humans, Asari, Geth, and every other race through the figurehead character that represents each race. ME2 made me care about the galaxy because I cared about the people in it, not idealistic ideas about justice and what is right for the galaxy. The crew of the Normandy is the best the Galaxy has to offer for a reason, to put it like Javik, the "Avatars" of their race. And so yeah, while I *LOVE* ME3, I wouldn't have without ME2. And I wouldn't keep going back to play ME1 if it wasn't for ME2. Why do you think people cared so much about the endings and closure? Why do you think people loved Citadel? Why do you think that ME3 was voted the most anticipated game of the year?
Because we cared about it. Because we cared about the characters.
So like I said. While ME1 made me want to play ME2, ME2 made me want to WIN ME3.
And just as a bonus. what do you think the best moment/mission in the series is?
For me, it's the Suicide Mission as a no-brainer. Or specifically, every time the Suicide Mission OST track plays.