You know, I have difficulties in answering this kind of question...
I believe that videogames are (by now) a diverse enough media that prevents a direct comparison between some games.
Surely, we can only compare things that are different (there´s no point in comparing what´s equal), but there has to be a sufficient common ground. For instance, comparing Mortal Kombat and Street Fighter, Doom and Quake, Virtua Fighter and Tekken, Final Fantasy and Phantasy Star, King of Fighters 2003 and Marvel vs Capcom 2, Incewind Dale and Baldur´s Gate, Final Fight and Streets of Rage, Mario and Alex Kidd are viable scenarios, because all the titles have very similar proposals. There´s enough common ground for a solid comparisson that can rely on many factual and tangible aspects, and not relegated to purely subjective matters.
Maybe in the 70s or the early 80s, when games were very simplistic and similar in styles and objectives (at least on arcade and consoles) this question could be more easily answered.
But not anymore.
What is your favorite book? Or movie? And what is the best book you ever read? That best movie you ever watched?
Take note that these are two separate questions. One thing is one product being declared as the best, and therefore superior to all the rest. Another is being your favorite, since it can rely on pure subjective aspects.
For instances, a certain aunt of mine does love the movie ghost (the one with demi moore) Because it was the first film that the love of her life watched with her at the movies. He´s now deceased, but the movie is a reminder of him and the moments they shared together.
The fact that she likes the movie so much has a lot more to do with aspects of the movie that resonated in her life than the quality of the movie itself.
Same thing can happen when we have a new exposition to a genre. it creates an imprint on us. For instance, for any slasher movie fan the first one he or she had contact with probably had a huge impact. Even if the movie was of lower quality when compared to others, or even a rip-off that plagiarized a much better thing, since it was the initial impact at that certain person it will probably carry a lot of weight for this individual. After all, it was the movie that sparked interest in the genre for that person.
Movies and books have very disparate objectives. Some are done to make people feel fine, others, just to cause the audience to laugh. Some, are done in order to shock people, or maybe even contest certain values of the society. some can be studies regarding a certain historic topic or a human figure.
How do I compare directly a play by Shakespeare and an Agatha Christie mystery tale? A biography of a certain person of interest and a tolkien style fantasy book?
let´s get even more dramatic:
Which is best? An Austin Powers movie or the godfather? A teen movie like porkys/ american pie or hard sci-fi like 2001?
They all have very different proposals. And I could say that all of them were immensely successful inside the standards they established for themselves. They were all great movies.. given what each one desired to achieve.
A teen comedy can have lower standards, but as long as it can deliver what was ambitioned, it´s good. It´s no use setting high standards for oneself and then failing to deliver.
But, can we affirm that one of these movies is better than the others? I don´t think so. There´s way too much disparity among them. It´s like trying to compare a Disney cartoon to a Tarantino movie.
Surely, I might be able to tell that a certain movie was the one I enjoyed the most during a certain period, but to say that one was better than others from different genres and totally different proposals sounds a bit nonsensical to me. A cargo train is no better than a racing car or a farm tractor or a 747 plane. And no worse either. They all serve different objectives. Each one fulfills different functions. They are all created with different applications in mind. Each one is good and appropriate in its own environment. There´s no point in trying to make a direct comparison between them. They don´t compete with one another directly. We could, however, draw a direct comparison between two planes designed to take passengers. Or two subway systems. Or two racing cars as long as they belong to the same category.
I see no point in comparing Pac-Man to Samurai Shodown 2. Or declaring the better game between Super Mario Bros 1 and World of Warcraft. Or trying to directly compare SimCity 2000 and Unreal Tournament. Or tekken 5 and tokimeki memorial.
I could certainly point what are my favorite games among certain genres, or some games that don´t fit a certain genre. or say what games entertained the most during a certain period of time.
I can say that I like Mass Effect quite a lot. Even when considering several shortcomings of the trilogy. The good aspects are so good that they allow me to overlook several bad aspects.
Is it the best game I ever played?
I don´t know.
And to be honest, I do not think I care.
It suffices to say that I enjoyed it quite a lot
But I don´t think I would say that I like Mass Effect more than Super Mario. Or Real Bout Fatal Fury. Or Simcity. Or mario kart.
Each of these games has a certain appeal. Each gives me a different kind of pleasure. The satisfaction I obtain from playing one is quite different than the good feelings I get from the others.
I can say, however, that the series has a lot of replay value. It´s immensely valuable in this regard. Either from gameplay perspective with different classes and specializations, or the branching storyline. It´s a game that I find myself coming back to from time to time. After all, there are still so many combinations, so many permutations I haven´t tried yet. And it´s also one of the very few games out there that made characters and history with a lot of emphasis.
One of my all-time favorite game/series? By all means.
The best game of my life? Don´t know. Who cares? I certainly don´t. And see no point in trying to determine it.