Mass Effect 2 was by far my favourite, followed by Mass Effect, then Mass Effect 3.

ME2 has the most character development, the most characters you can have in your team, the most influence you can have on the Normandy, e.g. Getting upgrades and scanning planets, and very gripping action from start to finish.

There was a wider range of romance options, and the chemistry in the team was well thought out. The return of old favourites from Mass Effect was a joy to see.

As for Mass Effect 3, i found it too short, a lot of the features from the previous games were removed, the dialogue options were limited, and above all, there were no krogan squadmates (without DLC).