It's an interesting question.
I am in the minority in that I didn't like ME1 very much. I didn't even get it until ME2 had come out only so I could understand the context. My biggest complaint about ME1 is that gameplay is just not as good as ME2 and ME3. The Mako is just an absolute CHORE to use, all of the planets feel exactly the same, all of the dungeons literally are exactly the same, and the game overall is just SLOW. I think I spent about an 1/8th of my playthrough in an elevator. The cover mechanic is half-assed and most egregiously, your powers just don't feel very special. I played through most of the game just using my guns because they were more effective than bothering to pause the game and use a power with an 11 minute cooldown just to lift one half-dead dude up into the air. Also, the difficulty curve went out the window halfway through the game once you got a Master Spectre Weapon.
Storywise, I think ME1 is on par with ME3. I personally never quite got the ending of ME1, because you spend the entire game looking for the Conduit only to find out that the Conduit is a backdoor onto the Citadel. The entire game you are told that Saren needs the Conduit to summon the Reapers, but that's not true. He needs the CITADEL to summon the Reapers. The Citadel that he already HAD access to (Almost unlimited access btw since he was the top Spectre). So rather than bothering to attack Eden Prime in the first place, why didn't he just wait on the citadel and have Sovereign and his Geth fleet come through the Mass Relay and in the confusion sneak to the Citadel Tower and activate it like he did in the end? Saren would have never needed the Conduit if he had just you know, been smart and realized that he already HAD access to the Citadel.
Squadwise, I also really disliked both Ashley and Kaiden so much. I wish there was an option to nuke them both on Virmire.
ME2 was a dramatic shift for the series, both gameplay and storywise. The gameplay was such a significant increase over ME1 I couldn't believe it. For starters, they greatly improved the cover and shooting mechanics. You couldn't just derp around with an assault rifle and roll face anymore. Getting in and out of cover felt a lot more natural. Guns felt different enough that playing with for example a Viper Sniper rifle felt a lot different than a Mantis or a Carnifex over a Predator. Each class had it's own playstyle and unique power. Powers felt useful and different, and since they made different powers affect different types of defenses, you had a need for them all. Submachine Guns and Heavy Weapons kinda opened up the field combat wise for caster classes.
Storywise, the game was pretty much The Dirty Dozen in space. The focus was on your squad and your mission, rather than a central antagonist like Saren. Whether that was for the best, I dunno, but I liked it enough. I don't think I enjoyed parts of the story as much as ME1 and ME3, but I really enjoyed getting the know the characters, especially Mordin, Thane, Garrus, Grunt and Tali.
Obviously, ME2 wasn't a perfect game. While I did not bemoan the skill tree changes as much as some (I personally thought that it was for the most part a good redesign) I did wish that there were more abilities available to you (See ME3). Also, Scanning was dumb. It just was. To paraphrase Egoraptor and his wonderful Castlevania video:
What is the difference between:
A: Killing a bunch of enemies, to get to a research upgrade plan, then scanning a bunch of planets to get the resources to research that upgrade
and
B: Killing a bunch of enemies, to get an upgrade
Answer: 4 HOURS.
ME3 has a lot going for it, despite the ending. The story is well done; there are parts in ME3 that got a emotional response out of me that blew anything ME1 or ME2 had out of the water. Mordin's scene on Tuchanka especially. Also Rannoch's story was very revealing and well done. That isn't to say it's not without it's flaws. The ending was an obvious bungle. Kai Leng was stupid and didn't need to be in the game.
Gameplay wise, I think ME3 is the best of the three. It fixes everything I had a problem with in ME2 and improves on other stuff. Upgrades are simply found or purchased, scanning is far simpler and less time consuming.
If I had to rank them for story
1st: ME3
2nd: ME1
3rd: ME2
If I had to rank them for gameplay
1st: ME3
2nd: ME2
3rd: ME1
