Played the originals in a shitty PC back in the day. So I'd guess the legendary edition or whatever remaster would be a good start. And now: my absolutely biased opinion.
Background: I'm a huge classic single player RPG nerd (Chrono Trigger, Final Fantasy IV, V, VI, Harvest Moon, Seiken Densetsu, Dragon Age Origins, Isometrics like Shadowrun and Pillars), love me some D&D, Pathfinder, Shadowrun, and board games with coop and/or interactivity. Do not enjoy grindy multiplayers, and/or games that require more reflex or inputs, I like to prepare for a journey and enjoy a good story. Loved the original Dragon Age, disliked WoW, like the idea of Dark Souls but hate to pilot it.
ME1: My favourite on the series. My first playthrough was like Dragon Age 1 in space. Like in charge of my own RPG avatar going on a huge quest in space. Felt the most similar to a TTRPG. Loved to explore, loved the characters, loved the depth of the setting, sorta liked the combat but was absolutely secondary to the enjoyment of discovery and setting. Felt like playing a TTRPG with friends and a good DM.
ME2: Distant second, loved some individual storylines and the DLCs. Hated the more "action" oriented gameplay and the implementation of "consequences" felt weird to me. A bit too scripted. A bit too "grim" and "edgy". Never felt really in charge. Some characters were absolutely awesome, but didn't "connect" with any of them. Felt like playing a really complex D&D module that had individual interesting adventures and NPCs, but was mostly building up to one final scripted encounter.
ME3: Played some after a while since my PC at the time couldn't run it. Didn't click at all. Felt like I was being led into a story using a sort of clone of my original character. Did not get invested at all. Maybe I'll play it again some day just to finish the whole epic. Felt more like a bad D&D campaign, so I quit it early.
MEA: Haven't played it at all, but feel attracted to the idea.