Ugghhh, are people complaining about ME2 again? ME2 is actually probably my favorite, and the best, in the series. Really the only consistent complaint I see from people is that ME2's plot was "bad," while to me it was perfect. ME2's plot is only retroactively bad because of ME3 and its decision to make everything you did in that game almost completely irrelevant. But, at the time it came out I thought the story in the game was FANTASTIC. You see ME2 was supposed to be the Empire Strikes Back of the series, more focused on characters and world building less about advancing the actual main plot. In Empire, the best SW movie, nothing really happens at all. Han and Leia dodge the Empire the whole movie and Luke trains on a swamp planet, then at the very end they all meet at some unimportant spot and it ends on a cliffhanger. The actual main plot is almost non-existent but that's fine because the story is all about the characters, it's about Luke learning what it means to be a Jedi and confronting his fears. And, it's about Han and Leia realizing the feelings growing between them. It is not about confronting the Empire or figuring out how to stop them.
The same thing happens in ME2, you have a paper thin main plot which is always present, Stop the Collectors, but the real meat of the game is the character building and exploration. ME2 is the developers giving you one last chance to poke around in the world of ME before the Reapers show up and destroy everything. Funny, while the Mako was taken out of the game ME2 actually feels larger than ME1. You have four main hubs to explore Citadel, Omega, Illium, and Tuchanka, alongside countless sidemissions you can encounter on various world. And, unlike ME1 each of these worlds, whilst condensed to a single area, feels unique as opposed to the copy/paste system of ME1 that tried to create an artificial sense a larger galaxy. ME2's story isn't about the Collectors it's about exploring Tuchanka and the plight of the Krogans, it's about learning Quarian culture and the inevitability of war between the Geth, it's about Blasto.
All of this "side" content is supposed to get you more invested into the world of ME so that when the actual Reaper war happens you have more of a connection to these worlds being destroyed and the conflicts that will have to be resolved. ME2 is actually the closest game to what some critical fans have asked for, a space adventure game that isn't about stopping a galaxy ending monster threat. While the Collectors are a threat they aren't something that must be stopped straight away, in fact the game encourages you to take all the time you need before attempting to stop them. Thus, ME2 feels the closest to a type of Star Trek like game. While you have this main threat you'll eventually have to deal with you are free leisurely to explore the galaxy, answer distress calls, mine planets for resources, chat it up with old friends, become involved in space politics, etc. Thus, every "side" mission feels like it's own contained episode that further fleshes out the world, but also links back up to the overall goal. This is in contrast to ME1 which while seeming bigger is at its heart a "chase" story, so it doesn't often make sense for you to leisure around the galaxy when Saren is out there getting closer to unleashing the Reapers.
So that is why for many reasons I love ME2 the best.