This has been discussed many times, but ME3 IMO wasn't good game despite the ending. It's campaign was really uneven experience.
Even beginning left lot to be desired, that's a lot to do with ME2 though, that didn't really advanced the plot. Original lead writer Karpyshyn had idea for the trilogy ending which ironically got partially used by Hudson because Walters couldn't deliver.
From beginning we get to Mars which was somehow shallow, also the first silly 'cinematic' scene where bot runs from you. Well, so what if character happens to have biotic charge, but you can't. What about stasis or lift? Nope, because we must have this cinematic shit here.
In other hand the Citadel was very good. It changed during the story, dialogue changed, new NPC's popped up. Mostly very good except stupid story line about Cerberus getting platoon of soldiers inside trough kitchen door without anybody noticing and again forced cut scene stupidity.
For main campaigns Genophage plot continued from ME2, which was IMO it's better parts. I think it was written by Patrick Weekes in both. Geth/Quarian storyline was alright to passable, but it was downhill from there. Last missions before Earth are on Cerberus installations, which latter was quite empty and outright bad experience, but we get to kill Kai Leng. Design is similar to Asari monastery side mission. Lot of potential but then nothing really happens until the end.
I think main content really suffered from having Omega cut as DLC*, which then didn't really integrated into main campaign and that everyone was expecting to fight the Reapers but instead it's Cerberus everywhere, which started reducing them to cheese faction from the beginning. Really poor choice to have two final stated before Earth on their facilities. If only we had campaign arch where we fight Cerberus missions, to point of Omega, where we get location of Cerberus installations, raid them and are done with that. For the rest of the campaign Cerberus could be replaced by Reaper troops.
But that's not what we got but game with horrible pacing issues, which was buried in negative feedback due ending. IMO TIM on Reaper Citadel was just final insult to injury.
Dragging Citadel to Earth was another huge mistake. For those who played through the series, Earth was quite distant and IIRC didn't even appear but one of default backgrounds in ME1. Getting it dragged into mess was unnecessary and meant losing everything players really connected to. Terrible choice.
For end game, I think it was even in Karpyshyn's original vision, Biotics. It's hinted many times and Javik says it outright, Asari were supposed to be the solution (but again DLC) but Reaper don't really have good defence against Biotics but shields. Corrupted Asari are only biotic option they have and those are few. What I might have been done might have been digging corporations from ME1 back to stage and joint development of better biotic amplifiers. So fleets had Thanix cannon tech for offence and then for defence some ships could have used at least partial biotic barriers with personnel trained for that. Thanix enhanced by warp ammo like properties is interesting idea too.
In ME1 properly build Liara could temporarily fuck up entire Geth platoon including their heavy tanks.
IMO, it's not about winning but that going by original vision, Reapers weren't robots but AI's which mission was to find solution for dark energy spreading in galaxy. It doesn't matter what it was but they had a mission, AI's can be negotiated with especially when in position where Reaper losses during this cycle would compromise their primary mission.
Doesn't matter really but that they had pieces necessary in the series already without needing to pull Crusible from their asses. In worse scenario they could had used 'cut scene powerful' cheese scenes like in ME2 and even that might had been excused by audience but they didn't see it as opportunity, nor wanted to do it.
* I was stupid enough to pre-order the collector's edition which came with short comic book. It's quite evident that Omega was originally supposed to be part of the main campaign.
