I thought the plot of 2 defied belief. It turned Shepard into space Jesus. The only things that saved that game were some of the new characters, the hilarious conversations, and the complexity of the endings. For some reason, people really didn't like the combat in 1. I think it is the best game of the franchise overall. To each their own.
To be fair, ME was never super much about the overarching story to me. It was always about the characters, and their interactions. The Reapers, Cerberus, etc. were mainly there to provide enemies and something to drive the plot forward for more character developments and interactions. And yeah, the combat in 1 is better than a lot of people give it credit for nowadays, but of the 3 games it has aged the most poorly, in my opinion. That doesn't mean it's a bad game, just that it tried some things that I don't think most people are going to like going in expecting a modern third-person game with shooting and RPG elements, when it's a bit weirder and dated compared to that expectation in the current gaming industry.
Honestly, I think all 3 of the games are good, 2 is just my personal favorite. It felt like it had some of the best characters and interactions, to me, and that's always been the core of what I like in a narrative. I don't mind if your overarching story is kinda lackluster and not very original if the
characters are compelling and interesting. You can have the most interesting plot idea in the world and stick a mediocre blank slate character in the protag seat and I won't get anywhere near as invested as an alright idea with characters that feel interesting, three dimensional, and have genuine thought and feeling behind them.