It's tough to write a review for Mass Effect, because, as cheesy as it sounds, words barely suffice. Mass Effect is almost like a religious experience for me, even though I'm an atheist. It allows you to transcend yourself and feel part of the greater universe.
As someone who wonders about the deepest questions of life, Mass Effect rubs me in all the right ways. I mean, isn't it strange that life and the universe exist? We are all here, but nobody really knows why or even just how yet. In Mass Effect, you will not only wonder, but you will actually find out!

Mass Effect has been my favorite game series, ever since I first played it over 10 years ago. It’s the only singleplayer game that I’ve replayed multiple times. It shattered my prejudices against science fiction, and as a result introduced me to some spectacular books like Hitchhiker’s Guide to the Galaxy, and Asimov’s Foundation series, and made reading fun for the first time in my life. It’s given me a future to dream about. Mass Effect feels like a glimpse into a possible future where we'll be traveling amongst the stars. Except, I don’t think FTL travel will ever be possible.

The main theme in Mass Effect is artificial intelligence, but it also explores deep political themes, e.g. how does each race organize their society? Is birth control (ever) the right solution?
The Paragon/Renegade system forces you to make hard decisions. What are you willing to sacrifice? Consequences may carry over to the next game, even all the way to the finale.

Mass Effect is also one of the best in world-building. As you walk around, you’ll often hear people talk about their work, how they view other aliens, etc. Side quests actually contribute to the lore.

That’s the greatness of Mass Effect, now a little bit about the individual games.

My favorite, and for most people the least favorite. ME1 is a proper introduction to the universe.
Here you can drive on tons of exotic exoplanets (for sidequests and extra credits) each with their unique atmospheres, terrain, and sometimes wildlife living on the large plains and tall mountains. It’s awe-inspiring. The main missions are also much more memorable for me than those in ME2 and ME3, because of the unique environments. (It's the side missions that most people find boring/repetitive.)
In ME1 you learn about the deepest secrets of the Mass Effect universe. The truth keeps getting worse as you find out more. The hallmark of a well-written story.

The prologue and finale of ME1 surpass the other two imo. The bombastic intro sets the stage for the whole trilogy, and the final boss fight was very disturbing. Also, the intro and credits songs are the best pieces of music in the series imo.

My least favorite, and for most people the favorite. ME2 moves the spotlight towards the shadier parts of the universe, where the law doesn’t reach. I don’t like that a lot of the game takes place in these forgettable poorly-lit corridors, and I don't like having to deal with criminals and mercenaries. It feels weird to be able to convince some of them to change their criminal behavior and lifelong-held beliefs with just a couple Paragon-flavored words.

In ME2 new alien races are introduced along with some 10(!!) new characters. I think the sheer amount of characters is not really a good idea. Combined with the freedom to choose your squad for any mission, the result is that characters don’t have as much dialogue.
There is one loyalty mission for each character. But 1 mission seems very little to gain loyalty of someone. Also, considering how personal the issues are in the missions, I always wonder how can I trust them to be motivated to fight for the fate of humanity? The characters in ME1 seem to have much greater ideals, and you also spend more time with them.
But I admit the loyalty missions are very well written. They all explore some ethical question. Mordin and Samara were my favorites. They were often (un)intentionally funny, because of how radical their views are.

ME2 is much more difficult than ME1, because you don’t have infinite ammo anymore. And the enemies have become more resourceful. They throw grenades now, with 100% accuracy.
The Harbinger enemy always creeps me out, as he slowly walks up to you with huge HP and a very devastating biotic attack that stuns you even when you're behind cover.
I remember when I first played 10 years ago, I'd die a lot in ME2, sometimes like 20-30 times in the same area, and the final Suicide Mission was the only moment in the trilogy I had to lower the difficulty. But playing the Legendary Edition, it was mostly smooth sailing. So you do need some experience in shooters if you want to survive, but it's actually not the hardest game.

I highly recommend people to play the whole trilogy on the highest difficulty, Insanity. For me, it really added to the feeling that you are struggling against the most powerful forces of the universe.

Nobody’s favorite. But still great in its own right.
The writing is slightly less compared to ME1 and ME2, I guess because the lead writer has been replaced. Shepard feels a bit out of character at the start, but you get used to it.
In ME3 they tried to make things more emotional, but it doesn’t work very well. With how many millions of people have died so far, why am I now supposed to care about this one child?

In ME3 there are almost no more great mysteries, but still a lot of interesting backstory. The Leviathan DLC is really the best DLC of the trilogy. The planet you’ll land on is unlike anything else, terrifyingly vast, in line with the general vibe of Mass Effect. In one of the main missions, you uncover some interesting secrets of Asari culture.
The ultimate fate of many species will depend on your choices in this game!! along with some choices made in the previous two.

ME3 is not as mysterious as ME1 or diverse as ME2, but it is the final climax. The ending is guaranteed to blow you out of your seat, if not move you to tears knowing that you just experienced the most epic story in all of gaming history.

I honestly think people will still be playing Mass Effect in 2183, just as we are still listening to Beethoven now.
And some exoplanets are going to be named after this game’s planets. At least, I think that would be pretty cool.