I'd rate the series ME1 > ME3 > ME2.
Mass Effect 1 had the best combat. Even if it was janky and filled with slowdown. Just the customization options available to tailor guns and stuff to your tastes was fantastic. By the end of the game I ended up with assault rifles with mods that made them never overheat while pumping out hot death. The entire last level was just me holding the trigger and pushing forward. It was the sci-fi action movie that the game promised from the beginning. That and explosive round sniper rifles. Or explosive round shotguns.
But ME2 had no customization, like five guns in the entire game, and a whole bunch of damage sponge enemies. That and unless you were in cover you got killed pretty easily.
ME3's combat was greatly improved from 2, but still was nowhere close to the first.
As for everything else: ME1 made me feel like an explorer. It made me feel like an actual spaceman when I set foot on planets and explored shit. The atmosphere - even in the carbon-copy outposts, derelict ships and mines - was great. ME2 and ME3 were simply corridor shooters. There was no semblance of exploration or adventure, just the notion to push forward and to follow the marker on the map. ME2 and 3 are still good games, but everything that made ME1 unique was streamlined to hell. EA's influence was all over both games.