I know it's controversial to have the very model of a scientist Salarian so low down the list, but let me explain. First off, they couldn't all make the top five. We'll all have our favourites, but aside from a few iffy sections in Thane's and a lack of creativity in Zaeed's, there's nothing wrong with any of them.
Obviously, the genophage is a huge part of Mass Effect lore. Some would even say it's the single defining factor in revealing the complexities at the story's heart. But despite its presence in Mordin's loyalty mission, the truly heartbreaking part of this narrative comes in Mass Effect 3. It had to be him. Someone else might have gotten it wrong.
Because of that, the weight of the story counts against it a little. The first time it felt like you were burrowing to the core of this controversial science, but on replays it just feels like a prologue for the really heavy stuff in Mass Effect 3.
Gameplay wise it's solid, but there's nothing to elevate it above tenth. Besides, there's a better mission on Tuchanka anyway, as you'll soon see...