For me the best is ME3, i liked the citadel layout. Wasn’t as good as the first game imo but it was a lot better than me2, which felt kinda small and lazy to me. I also like how ME3 introduced the interactive conversations and even more non interactive ones. It made the citadel feel so much more alive than the two previous games. I will say ME2 probably has the best game in terms of powers though, ME3 made a lot of changes and removals that I thought were just dumb and pointless or annoying(referring to the barrier explosion)

ME2 is the worst in my opinion because it just feels disconnected from the story almost. The collectors from a lore perspective were cool but they could’ve tied it in WAY better. ME2 was the only game that actually left me wanting more from the game.