I never finished the third one. I kinda want to revisit and finish the trilogy sometime. I remember enjoying the RPG elements but looking back on it, it really glorifies the military, you're basically the space CIA if I remember right. You also team up with Garrus, who's a vigilante cop and the Space Fascist in the third one, whose genocidal racism is treated like "haha he's like your racist grampa lol".
I liked how much more open the first game felt.
And it made sense with the fact that the story is essentially an investigation into Saren and his dealings
With 2 the story starts getting real wobbly, what with forcing you to work with Cerberus because (insert reasons here) and the story basically just being "You need dudes to save everyone"
3 is just gibberish outside of a few particular quests and please don't get me started on the ending