Mass Effect 2, because in my opinion of the three games this one´s the most successful at achieving what it had been set to achieve.
It provides the most polished experience, comes with a solid combat system (better than ME1, worse than ME3), features the most robust dialogue system and has the best character writing. It´s at its best when putting players into recruitment/loyalty missions which happen to be the focus of the game, culminating marvelously during the final mission.
In my opinion the biggest pitfalls of each game would be:
ME1 - unsuccessful attempt at marrying RPG and shooter mechanics leading to the game being incompetent at both, inventory system, repetitive planetary side missions, abundance of fake dialogue choices (choices leading to exactly same things being said)
ME2 - core plot being irrelevant within the context of the trilogy, RPG mechanics being oversimplified, planetary scanning
ME3 - bad choice of lead writer resulting in haphazardly put together core plot, final combat mission, lack of reactivity to ME2´s final choice
Some of my favourite elements in each game would be:
ME1 - the way(s) it introduces its universe/lore to players, core plot, Virmire mission which delivers a great mix of gameplay, plot development and player involvement
ME2 - overall level of polish, soundtrack mainly during recruitment missions (develops as these progress), the final mission featuring a fairly unprecedented amount of variables stemming from players´ actions (a rather weak conclusion of the core plot notwithstanding), Legion and Mordin characters which I consider the peaks in the BioWare´s catalogue, variety of settings at times successfully building upon lore laid out in ME1 (Tuchanka or quarians) and laying foundations for ME3
ME3 - combat system, Normandy which is probably my favourite iteration of a player base to date, the conclusion of the genophage story arc that thanks to its writing and variables involved is in my opinion the biggest BioWare´s RPG storytelling achievement to date as well as one of the best of its kind within the whole genre, conclusion to the Conrad Verner story arc which, despite its briefness, may as well be my favourite easter egg-y content in a game
All three deliver mostly excellent voice acting (particularly when the sheer amount of it is taken into account) and I consider the possibility to pick between male and female protagonists to be the series´ notable asset, particularly because it leads to a very different experience, even though the lines are mostly the same.
All three games also come with overall excellent soundtrack. I feel ME1 uses/introduces the biggest amount of memorable themes, while ME2 is the most successful at incorporating music into missions. ME3 is probably weakest in this regard but still very good.
As a perhaps unusual plus I´d add that I rather appreciate that all three games belong to the same "gen" audiovisual-wise because it helps making the transition between the titles smooth, resulting in it being fairly easy to imagine the trilogy as one game.
I´d also mention that story DLC for ME3 (including the extended ending) in my book transform the game from the weakest of the three to one that I find as enjoyable as ME2.
With the inclusion of DLC I score all three games within 8-9/10 range, for often different reasons. I do so with taking what they´re attempting to be in heavy consideration, rather than mulling over what they´re not. The uniqueness of the experience provided plays an important role as well.