For me, ME2~ME3<ME1. The first one was unarguably the best. For all of its glitchy poorly ported unfixed-to-this-day glory, it also had the best music, best immersion, best love scenes, some of the best character development (ME2 only had real merit in developing Jack and Garrus, the other characters were mostly static, their development happening off-screen between 1 and 2 or between 2 and 3; while ME3 only developed Tali, Legion, Garrus and Javik; for all the others the development is once again off-screen, on-screen they are static), real if a tad limited planetary exploration, and, of course, it felt a lot more RPGish. It's ending and overall impression makes you want to look at the stars in the night sky with hope, apprehension and that tingly shiver running up your spine as if caressed by a gentle lover.
ME2 tossed out the number-crunching and inventory and the Mako, the sex, the electronica, the faux-retro style (I fully support the Decades Of Design concept -- ME1 is the 1980s, ME2 1990s and ME3 is the 2000s -- as it is a good way to explain all the inexplicable design changes), threw in more characters - most of which were brilliant, don't get me wrong, just lacking significant depth - and meandered a lot with the plot. It was never about the Destination as much as ME1 was, it wasn't even about the Journey, it was about the Companions. It does, however, retain The Hope and the Love For the Stars, even though you will now more cautiously look at the night sky, and get not shivers but goosebumps.
Similarly, ME3 tried to return partway to the roots - the weapon mods, the smaller number of squadmates, the (failed) attempt at restoring sexual and same-sex elements, the Destination. It is mostly about The Journey, because the Destination turns out to be a one-way trip to Plotholeville.
It does a lot of things awesomely, and resolutions to most of the mid-game plots are cool, but at the same time it is severely railroaded, discards all your decisions in what is functionally the Final Dungeon and all in all seems to focus too much on violence, gore and death, forgetting that the things that made the first two so enjoyable - The Destination and The Companions are as important as The Journey. Thankfully, it did not ruin my Love For the Stars, but I am an astronomy nut, and I might be looking too much into it =)
TL;DR: ME2 and ME3 are equally good because they both squandered most of their potential away, ME3 far more so than ME2, because it had much more going for it at the start; so ME1 comes out on top simply because it has a higher percentage of Expectations Fulfilled (about 130%, because surpassing KotOR was easy after KotOR2 showed how to do it, and surpassing KotOR2 was easy because it, too, had a frelled-up ending; conversely ME2 has 90% (Tali's Unreveal, the lack of sex, the lack of exploration, the reduced immersion because of elevators and airlocks being turned into teleporters; ME3 is about 75% because of railroading, lack of exploration, Tali's Second Unreveal, lack of closure, lack of real ending, Kai Leng being based on Deception and not Ascension, among other things).
TL;DR was TL;DR:
ME2=ME3, ME1 much better.