ME2 redefined combat for the ME franchise by introducing the class defining powers Charge, Cloak, Drone, Tech Armor and Adrenaline Rush.
I have a hard time agreeing with ME2 being the worst game based on how much of an impact it had on combat mechanics alone.
Mass Effect 1 already did bring class defining powers, although not all classes had them.
Only the Soldier had combat armor
Only the Engineer had hacking
Only the Adept had Singularity
The other three classes were hybrids and lacked a unique power in ME1. The power that originally was the most innovative, unique and defining for the Mass Effect universe has been Singularity, already present in ME1. I'm not saying the ones in ME2 were not welcome, nor that some of them were not innovative, nor that they were not defining for the rest of the series. The changes to singularity alone were great, especially since it was the first ability that got the option to explode. Charge is awesome, Combat Drone is nice. The others, although good, aren't really that innovative imo. Many games out there have a cloaking ability. The real useful feature is the slowdown that comes with, but that is not really that innovative either. F.E.A.R. had slowmo. Same applies for Adrenalin Rush. Tech Armor is basically Armor Boost from ME1 with a discharge/explosion feature. And looking at it from this perspective, it's still ME1 that is the basis for everything.
All that being said... The most important part is this... Combat is not the reason I play Mass Effect, and I doubt many people play it for that reason. If it was, Andromeda would have been the most successful Mass Effect game. It wasn't, and, that's why I find it kind of a hard sell to use combat as the main argument as to why ME2 is the best. Andromeda went the same way as ME2; improved combat at the cost of practically everything else. At least ME2 could compensate with great characters, but, that's really all that ME2 had to offer.
If we can learn anything from ME2, it's that people value anecdotes more than anything else. Both ME1 and ME3 do so many more things in a superior manner, but, because ME2 had nice moments with side characters (yes, the majority of your crew are side characters), people regard it as the best game in the trilogy, despite all the absurdities the game provides. The value of anecdotes is so high, that even the flaws were turned into memes that work for the popularity of the game rather than against it.
Firstly, anything that does not move the main plot forward is a side element, be it a side quest, side mission or side character. Taking ME2 as an example, the plot is stopping the collectors from abducting humans. Whatever action you take that improves survival rates of your crew or whatever are nice to haves, but not relevant in terms of plot.
That would mean that Mordin is a main plot character, because without him, it wouldn't be possible to be invisible to the swarms, and Horizon wouldn't happen. He was vital to the progression of the story. His loyalty mission is a side quest though, just like all loyalty missions.
Miranda would be a main plot character since she brought you back I guess, but technically it wasn't required for her to be a squad mate. Still, she's a main character.
Jacob is an introduction to Cerberus, nothing Miranda wouldn't be able to do, so, he's currently a plot character technically, but an unnecessary one and functions more as a side character.
Garrus is a side character.
Grunt is a side character.
Jack is a side character.
Samara is a side character.
Tali is a side character.
Thane is a side character.
Legion is a side character.
All DLC characters are side characters.
EDI is more important than the majority of your squad mates... And so is TIM.
People love to say that the ME1 characters were bland and shallow, but they had a clear purpose to the main plot.
Ashley/Kaiden serve as the trigger for getting the vision from the beacon.
Tali serves as evidence to show Saren is guilty and to allow you to pursue him.
Liara serves as an expert on the Protheans to help you decipher and interpret the visions from the beacons.
Garrus, even though he doesn't forward the plot, functions as the embodiment of frustration towards the council and C-sec regarding the Saren investigation, i.e. frustration towards stagnation of the plot.
Wrex is arguably the only one without a clear influence on the plot, other than the conflict on Virmire, where his role of supporting you temporarily turns into stagnating you. And funnily enough, he's generally considered the most fleshed out squad mate in ME1. Because the anecdotal experience with Wrex is a lot more valuable than the actual real value of forwarding the main plot that the other characters provided. Funny how that works.
I still maintain the perspective that the downward spiral of Mass Effect started with ME2. It got accelerated because of the high praise despite the many flaws in many areas that did not and still do not receive criticism. The downfall of Andromeda in particular was trying to structure the game as ME2, making it more open and fail to deliver high quality characters. There was no delivery of the necessary anecdotes for people to value the experience, and this time, the memes worked against the game rather than for it.
Technically, Dragon Age 2 and Mass Effect 2 got the same treatment. For some reason, people recognize Dragon Age Origin's superiority, but not Mass Effect's. And people recognize Dragon Age 2's flaws, and have real trouble recognizing them for ME2.
and from my memory too 
