I love ME2, ties with the first game for distinction of my favorite.
I think what really sells me on 2 is something that a lot of people probably didn't like about it, but the overall story structure. The whole game is, basically, recruit ally, talk to ally, fix ally's personal problem, repeat 11 times, with an occasional break to further the main plot.
That doesn't sound very interesting on its face but each personal story takes place at different points around the galaxy and builds the world you reside in while also building the characters. Its a clever strategy and, by the end of the game, I've learned about Quarian internal politics, the schism in the Geth, old Asari monsters that still prowl the galaxy, Krogan rights of passage, the personal effects of the Genophage, entire space stations that act as a giant slum, and more general things like the lengths that people would go to unlock biotic potential or how one deals with dangerous criminals in the era of spaceflight. Each of these I learned personally; I saw, spoke, fought, and felt the products of all of these facets of the world. I didn't just have it told to me by squadmates.
This is all stuff that fleshes out the universe and makes it feel lived in. It gives Shepard et al a reason to fight. Conversely, the side quests in ME1 were more generic: slavers hiding on a planet, mining operations gone wrong, rogue VIs. Good stuff from a sci fi perspective, but the universe didn't exactly feel lived in besides a handful of locations where actual civilians reside. Further, each of those personal quests in ME2 is tied to the main objective, ie build up your team until they're the best strike force in the galaxy.
Additionally, the Collector threat hits the right sweet spot of "threatening enough to be important" but also "clandestine enough that you can take your time figuring it out" so it still feels real without making you feel bad for pausing to help your assassin reconcile with his son. The mystery of "who" is collecting humans is solved immediately, leaving you with "why", a question which is sufficiently answered at the end only to raise more questions which make the arc villains more interesting ("Why turn the Protheans into Collectors?" "Who is Harbinger to the reapers!?" "Why were they building a f***ing human reaper!?") these reveal bits about the enemy without taking away their mystique. Their motives are still unknown, their origins are still unknown, we've just been shown more of their terrifying power to whet our appetites.
I feel that, game-play wise, 3 was the best as it had the best developed gun and power system and levels that were designed semi-intelligently but the plot and characterizations were all bonkers. As far as 1, I feel like it delivered on the "RPG" part of "Action RPG" and had a serviceable plot, but it didn't develop a unique universe outside of dialogue with squadmates; most sidequests and even a lot of the main quests were "generic scifi plot #x" and did nothing to build the world we were saving. 2's combat was generic, and it has the least RPG elements, but I think 2's story fits the series the best.
I think what really sells me on 2 is something that a lot of people probably didn't like about it, but the overall story structure. The whole game is, basically, recruit ally, talk to ally, fix ally's personal problem, repeat 11 times, with an occasional break to further the main plot.
That doesn't sound very interesting on its face but each personal story takes place at different points around the galaxy and builds the world you reside in while also building the characters. Its a clever strategy and, by the end of the game, I've learned about Quarian internal politics, the schism in the Geth, old Asari monsters that still prowl the galaxy, Krogan rights of passage, the personal effects of the Genophage, entire space stations that act as a giant slum, and more general things like the lengths that people would go to unlock biotic potential or how one deals with dangerous criminals in the era of spaceflight. Each of these I learned personally; I saw, spoke, fought, and felt the products of all of these facets of the world. I didn't just have it told to me by squadmates.
This is all stuff that fleshes out the universe and makes it feel lived in. It gives Shepard et al a reason to fight. Conversely, the side quests in ME1 were more generic: slavers hiding on a planet, mining operations gone wrong, rogue VIs. Good stuff from a sci fi perspective, but the universe didn't exactly feel lived in besides a handful of locations where actual civilians reside. Further, each of those personal quests in ME2 is tied to the main objective, ie build up your team until they're the best strike force in the galaxy.
Additionally, the Collector threat hits the right sweet spot of "threatening enough to be important" but also "clandestine enough that you can take your time figuring it out" so it still feels real without making you feel bad for pausing to help your assassin reconcile with his son. The mystery of "who" is collecting humans is solved immediately, leaving you with "why", a question which is sufficiently answered at the end only to raise more questions which make the arc villains more interesting ("Why turn the Protheans into Collectors?" "Who is Harbinger to the reapers!?" "Why were they building a f***ing human reaper!?") these reveal bits about the enemy without taking away their mystique. Their motives are still unknown, their origins are still unknown, we've just been shown more of their terrifying power to whet our appetites.
I feel that, game-play wise, 3 was the best as it had the best developed gun and power system and levels that were designed semi-intelligently but the plot and characterizations were all bonkers. As far as 1, I feel like it delivered on the "RPG" part of "Action RPG" and had a serviceable plot, but it didn't develop a unique universe outside of dialogue with squadmates; most sidequests and even a lot of the main quests were "generic scifi plot #x" and did nothing to build the world we were saving. 2's combat was generic, and it has the least RPG elements, but I think 2's story fits the series the best.