By the power of truth, I, while living, have conquered the universe.
ME1 - 8.5/10
ME2 - 8/10
ME3 - 4/10
While ME1 had its problems with the clunky combat system, frustratingly bad inventory UI, repetitive environments and popping textures, none of these issues stopped me playing it 19 times through. It's easily the most immersive one of the three for me, especially when it come to side quests because they often interlink which gives your actions some consequence and reward. The Prothean Sphere on Eletania, for example, is something you can only access through the Consort's trinket - which can only be gained by doing Xeltan's quest too. It's amazing how instances like these reward you for doing all the extra stuff, some critical lore is learned, the mystery carries over to ME2's Firewalker missions, and is finally explained in ME3 - only you need to pay 1600 MSP for that particular mystery to be solved...
ME1 has some of the best missions built up through the awesome environments and atmosphere, Ilos and Virmire stand among my favourite missions of all time because of this. Ilos captured a feeling the rest of the series failed to replicate, it was a call-out to the mature, mysterious, alien sci-fi stories of the 80s which ME1 was so in-touch with.
And the villains? Saren and Sovereign are easily the best in the series, the former has some great dialogue and clear motivations, while the latter was absolutely terrifying. Sovereign had his own God-complex sorted out, you were nothing to him and the conversation you have with him on Virmire is unforgettable.
I stand as one of the few who liked the Mako sections on uncharted worlds, it could have been improved but the worlds were stunning and actually made you feel like you were part of a GALAXY, rather than the various shooting galleries in ME2 and especially 3.