Here's a different take where the series went wrong:
ME2 is the best game because it's the only one that focused on characters and world building instead of stupid giant space squids.
ME1 has massive flaws, bigger than 2 by miles. The open world sections were boring and repetitive, the combat abysmal, the powers boring and samey, the Mako handled like a drunk obese scotsman in a shopping trolley, the menus were user unfriendly, side missions were samey and charactdr interactions often shallow.
ME2 has a shit main plot and went too far with streamlining its systems.
Beyond that, it built on every single great thing about the first game. Player choice, consequences, world building, characters, emotive side missions, player agency and interactivity (god damn were interrupts awesome!) Customisation and personalisation while adding in far better DLC, especially with Lair of the Shadow Broker and ultimately having one of the most amazing, tense and exciting endings to any game ever with the suicide mission, giant terminator baby aside.
The worst problem ME2 had was not being willing to let go of the Reapers, which were a good set piece and background for the real villian of Saren in ME1, but set a precedent that each subsequent title not only felt oblidged to include, but inevitably raise the stakes on, ultimately ruining the 3rd game entirely.
The rot was laid in ME1 though, and while perfectly ok in isolation, ultimately tarnished a game that was primarily great for the universe itself, not the unknowablely vague, big bad squid monster.