Ask a Mass Effect fan to pick their favourite mission from the trilogy and you'll probably hear a range of responses. There's the Virmire level in Mass Effect 1, for example, where BioWare unexpectedly forced players to choose between two squadmates' lives. Then there's the superlative Suicide Mission finale of Mass Effect 2, perhaps the most intricately-plotted episode of the whole series. There are whole flowcharts to explain that one.
My favourite mission is Priority: Tuchanka, aka the one in Mass Effect 3 where you cure the genophage. It's a masterpiece full of character moments and a culmination of storylines which had been building since the trilogy began. Best of all, however hard you work, however many side-missions you have completed or what choices you made in previous games - there is no perfect ending.
Tuchanka is sometimes compared with Rannoch, a later ME3 mission where you resolve the similarly long-running conflict between the geth and quarians. Rannoch is great, with some similar emotional beats, but it has a clear "best" ending where it is possible (albeit difficult) to save both races. With Tuchanka there is no easy answer. The mission's choices run deeper, the consequences greater, and the writing is some of the finest throughout the trilogy. Tuchanka's ending also packs an emotional gut punch - depending on your choices it can seal the fate of at least one beloved character - including my favourite of the series, Mordin.