3 > 1 > 2
ME1 still has lots of jank but I admire the more ambitious scope, I miss the Mako, and I still feel it has by far the most concise and developed narrative of all three. ME3 really trims the fat and steers closer to being a shooter, but I feel that results in a more polished product. I love the combat environments, the upgrading system, and the scope of the adventure. The story and writing are quite good given the series and long standing plot arcs, only going truly to shit at the floating platform.
ME2 is ME2. I still love it, but it felt like a transitional phase. It has the direction of ME3, but is far to stripped back in most regards, and has the worst narrative of the three games (though I like all the mini stories). I wish ME3 had more subquesting in levels like ME2, but I can deal without.