I like Mass Effect 2 the most. It's still my favorite game ever. Both the first and third games would also make my top five (though it's too early to really call that on Mass Effect 3). I don't know how I'd rank them against each other, though.
As far as Mass Effect 2 goes, I love it every part of it. But most of all, I love the characters. It's a big squad with lots of interesting conversations. I also loved visiting alien worlds and learning more about their culture, as well as shadowy figures such as The Illusive Man and Aria. There were many improvements to the combat, though some systems were oversimplified. And the ending was just an absolute thrill, with actual life or death consequences based on your choices.
For Mass Effect...
The first game (which, again, is a top fiver for me. And it's a game I've played over a dozen times, with the vast majority of those playthroughs being mostly completionist) had a few things drag it down for me. I hated the Mako and exploration in general in it. I really don't see how anyone can miss that. Those secondary planets were dull and tedious, to the point that I'd rather scan planets than roll around on the Mako. There were some good storylines, but the method of reaching them was terrible. The inventory was the worst in the series, and I'll stand by that. Though there was a lot to manage, there was very little meaningful choice in managing it. There was almost always a clear-cut "best option", which meant 95% of what you find is just junk to sell. I don't have a problem with the combat in it like some do (I appreciate each installment's combat on its own merits), though Mass Effect's Insanity mode is easily the worst in the series. Instead of changing enemy tactics and aggression, they just made them all bullet sponges. It wasn't challenging, it was tedium.
But despite those issues, I love the first game to death. The tone had this amazement to it. I really felt that feeling of smallness in a vast universe. It also has the benefit of being where we were first introduced to the races, cultures, systems, and political bodies that inhabit the Mass Effect universe. I feel that every time I play it. And like I said before, I actually like most of the gameplay (basically everything that's not Mako-related). And the soundtrack was also fantastic.
For Mass Effect 3...
I only really have two big issues with this game, but they both hurt. One is the ending, which is a given. The other is Shepard himself. Our control over what he says is taken away far too often. Why did they suddenly do this in the third game? It was a terrible choice. Every time Shepard speaks without me selecting the dialogue option, I just want to cringe. Don't do that. Besides those two things, Mass Effect 3 is a wonderful game with a whole lot of best-in-series material. Best gameplay, best level design, best soundtrack (probably my new favorite game soundtrack. And I was highly skeptical of Mansell coming in. I actually derided the choice for him to do the soundtrack), best story-telling (until the end). It packed a whole lot of emotional punch and had all sorts of little tidbits that sweetened the entire package.
So to be more concise, I'll put it like this: I like Mass Effect 2 the most. I put Mass Effect and Mass Effect 3 as equals.