Methinks the original ME has the best story and the weakest gameplay, while ME3 has the best gameplay and the weakest story. ME2 is the middle child. It's story is better than ME3's but not as good as ME's, while it's gameplay is a big step up from ME but not as good as ME3. Though ME2 focuses largely on recruiting team members for the final mission and takes Shepard on a series of missions which are only slightly related to the main plot. ME2 feels more episodic. It still has better writing, and the stories are still interesting.
ME3 has a little more depth in the gameplay. There's more variety in the types of enemies you fight, superior level design, new mechanics are more depth to weapons and powers available. However, the story is the weakest. It's rather blatantly meant to wrap-up all the fanchise's plot points while the whole Crucible thing feels very deus ex machinima. It takes a big risk with the ending, which I can appreciate, but it backfires and comes dangerously close to turning the entire trilogy into a joke.
I would recommend you buy ME2, which is fairly cheap now. If you like it, you should buy ME3 later and continue your character's story.
I would say that I am somewhere in the middle of liking both games. With that being said, I would have to say that the pros and cons of both games are about even when you put it in terms of gameplay and story (plot). I will say that I am leaning towards Mass Effect 3 though only because of the online gameplay. I do wish though that the people that I play against would use their mic while playing to bring effective communication to the gameplay to make the online game last all the way to the end of the match (11th wave = extraction objective). Those that I have played with online seem to ignore the multi-point objectives that appear during a game.
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