There is no best part to ME3.
Mass Effect 1 was a good start to the trilogy. Introduced the story, worlds, lore and it was new and different and interesting at that. Sure, some game mechanics were pretty annoying, like driving Mako around the planets (was fun the first 2 missions but after that, sheesh...). Main thing was, players were making choices along the way which all meant something in the end. How you handled things, the missions, both big in story sense and small ones, they all added up in the end. And they shaped the end. Granted, the amount of choices wasn't that big yet but still, the players were building the result.
Then came ME2 and that was even better. Started off with a shock of seeing Normandy getting blown to pieces and Shepard dieing, but story soon evolved further and things moved on. Shepard was back and the Reaper cronies needed some kicking. This time things were even better, there were much more choices then in ME1 but they mattered even more. Many new chars, new locations, new ways of handling them. Many results which came over from ME1, some small, some bigger and which also in the end added to the final. In ME2 players really built the story from ground up, every action they did - or did not do - had a consequence at the end, had a result. One could play faster through the game and not bother with side stories and that had a result at the end. Another could meticulously follow every bit of data, every single mission and that again had a result at the end. Even the end had several stages where you had to make choices and again, those choices also meant a lot. Basically, as I said already, in ME2 the story was built from ground up by the player and everything reflected it, it was simply great. And game mechanics were also better. Combat was improved, new gear was added, like heavy weapons, more weapon options, customization of gear and so on. Everything what ME2 did was better. Even resource gathering was better then it was in ME1, not ideal but still better. So ME2 did everything right.
And then ME3 came out. Started off nicely again, with the Reaper attack and all. It was nice seeing the old friends from ME1 and some from ME2 back in action again, but right away things got annoying too. First, the heavy weapons were suddenly gone. They could only be used temporarily, they were randomly lying around in the world... So a good thing from ME2 was taken away in ME3, messed up. First negative impression. Then players started to also meet the crew members from ME2 and discovered that they were turned into trivial, useless chars. It's as if Bioware just wanted to do away with them. They didn't say much, or they were killed pretty fast, or they were a very little part of the game. Such a letdown, and another negative impression. Dialogue between current crew members is like some random sideliner talk, no immersivness at all. Only occasionally the dialogue with them was like "in the old days", where you could choose your responses. Basically totally dumbed down. Yet another negative impression. Relationships between the chars.. Makes one wonder what was the point of following the relationship with several chars since ME1. Ashley Williams was pretty useless, first appeared at the start of the game, then soon was gone for a while and when finally joined up, was pretty much pointless person. If you followed a relationship from ME2 then that seemed totally hopeless. Another negative impression. I have not yet played the saves where my char had a relationship with Miranda Lawson but seeing as she only appeared barely few times during the whole game, it seems to have even less point then Ashley Williams. Chalk up yet another negative impression. Relationship with Jack? She's yet another "cameo appearance person", so pointless again. Another negative impression. Tali'Zorah - can't say yet as I have not tried yet the saves with her, but she also appears pretty late in the game, it seems to be just as useless as Ashley Williams again. Another negative impression. Only one who had some integrity seemed to be Liara T'Soni, mostly as she's with the main char since the start. So all the talk by Bioware how "past relationships matter and how they will evolve in ME3" is completely pointless, it all is weakest and most pointless in ME3, almost a complete waste of time for anyone who thought that there will be more to it, or that these will be more involved.
So the game continues, delivering one disappointment after another. At the same time there are at least some good things too, for example seeing EDI advance into more sophisticared AI and pretty much into one of most advanced AI, who actually understands and has knowledge of organic. EDI seemed to become pretty much the vanguard of the real AI which would live together with organic life, understand organic life and at the same time, organics would also see EDI as equal and understand EDI and associate with her. Then all these historic moments, like curing of the genophage, uniting the krogan and turians, ending the quarian and geth war and laying the groundwork for their co-existence. There is all of this traveling around the Galaxy, building relations, gathering resources and allies, uniting the whole Galaxy to work together on the common goal, it almost feels like in ME2. You know, making the choices which in the end bring the best result. There is again the choice to not follow all these sidelines and move on with the main event, but there is also the choice to pay attention to every details, to get the seemingly best end result. Then you get to the end, play through it and realize that you haven't seen anything THAT BAD in such a long time, if ever. Anything you did, everything you did over the course of three games and several years matters nothing at the end. You pretty much get no choices whatsoever at the end. Doesn't matter how hard you worked over the course of several years and three games, none of that is considered at the end, none of that matters. There is absolutely no point whatsoever to anything done in all of the games before ME3. May as well just launch it, do the basic stuff and then finish it, the end is EXACTLY the same.
And your choices in ME3? Remember I mentioned how it was fun to see EDI evolve? Pointless, two choices out of the three simply eliminate all that work. Remember I mentioned ending the war between the quarians and the geth and laying the groundwork to the new union? Pointless, two choices out of the three simply eliminate it all, completely. And so on, and so on. And all of your work on your Shepard, making him stronger personally but also through his friends and allies is also pointless, because pretty much every choice simply kills him off. I just went through it and then was just sitting behind my PC and thinking: all these years, all these games, all this work and time what I put to meticulously following every single bit of info, doing everything I could do, making the right choices, preparing like never before, working in a game like never before and it all came down to this nonsense? And then you read some notes from the game producer telling how he is proud of what was done? Proud for ruining the game and basically ensuring that I will not play it again, will not buy anything related to it again? Proud for wasting all the effort over all these years, through all of these games? There is nothing to be proud of, you ruined what could have been one of the best and most "epic" titles out there by showing that the effort and loyalty of your customers over all these years and games mean nothing in the end.
And mechanics.. Cruising around with Mako was annoying in the end. Scanning planets was less annoying but still not the best. But the scanning in ME3 was totally dumbed down, silly. And multiplayer? Sure, it's a fun thing but actually forcing that to players is bad. Making it so that unless the player spends time in multiplayer, their single player game doesn't advance past a certain point is just rude. It's like spitting in the face and saying "Ha ha, don't care how much you work on all of this, if you don't do it the way we want you to do it, nothing will matter in the end!" Multiplayer could have been something which added bonuses to the single player game, instead of being something what had to be done in order to also advance some aspects of the single player game. Very annoying, very rude and very disappointing.
All in all, the quality of Bioware games is falling. Last titles have all been weak and incomplete (do I need to point out Dragon Age 2 for example?) and sadly ME3 faced the same fate. It had some new features which were better then in previous games, it had some new things. But it also removed the good existing things or made them totally trivial and pointless. And the game and story itself - was nice to a certain point, things seemed to go nicely up the hill, it felt interesting for a while. Until the end came. That was not a downhill path there, that was a fall into bloody endless chasm. You have nothing to be proud of Bioware, you ruined a good game.