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*gasps*
How dare he call Mass Effect 2 an action/adventure game! It's clearly an RPG!
If you slap RPG elements onto a pile of dog shit, it's still going to be dog shit. I am not offending anyone who considers Mass Effect 2 an RPG, because it is to a certain extent. But the primary gameplay mechanic is that of a shooter, or an "action/adventure" title. Yes, you have a "party" to choose from before you embark on any missions, just like in an RPG. Yes it is a "free roaming galaxy", just like an RPG. And there is stat building for you and your teammates, and as stated before, just like an RPG. But the game's premis is that of action, and of free roaming adventure. One can say all games can be classified as an action/adventure game, but that's a whole other topic right there.
Anyway, why is this the best action/adventure game? Besides the gorgeous visuals, excellent character animation, and fantastic presentation, the story is what's most important. A story in any medium, whether it be a book, move, or video game, is what hooks you in to keep reading/watching/playing. As Commander Shepard, you have been given a second lease on life; using some of the most advanced technology in your current era of time, you are recreated once more in your original form to tackle on a mission that is far greater than any one man or woman can tackle. It is humbling in a way, that in a galaxy of billions of humans, and multiple alien races, you are brought back to life for the soul purpose of saving the entire galaxy.
But you need a great team to do it, and this is where Mass Effect 2 shines. Sure you can pick the best soldiers/biotics/techs/grunts in the galaxy, but this mission you are preparing to embark on isn't going to be an easy one, let alone one that will have no casualties. This game thrives on the good leader concept; your teammates will fight with all that they have if they care about you. Most of the game revolves around doing side missions for your teammates, and forging bonds and connections with your crew. Thus, you get a sense that this is truly your crew, and you begin to care about them. 
Many games that feature teammates and interactable NPCs fail at this sort of connection. Any character, whether it is a main character or a sidekick, needs to have a struggle. Whether it is within, or external, there needs to be a dark, lurking presence within their souls that they cannot put to rest until some action is taken. It is through this kind of storytelling device that you, as the viewer or player, can forge a caring bond with the character(s). When that final, climactic battle looms in the horizon, you begin to fear for their safety, and you want to do everything in your power to make sure that they make it out alive.
How many games can you say made you feel that way? That when you saw your first teammate go down, you felt something press down on your chest in dread, like it was real life. Not many games, I'm sure.
It's an action/adventure title with a purpose. The gameplay mechanics, the cover system, and the voice acting are all top notch to boot, but the primary focus of this game is bridging that connection with your teammates before that last "suicide mission." And in the end, if you've played it right, you are so very elated to have everyone board the ship in one piece after your final mission. If you haven't, and some have suffered, you feel loss, just like any commander in the field would. Very few games out there have really expressed this kind of emotion to the player, and to me it's what makes Mass Effect 2 such a brilliant game.