What makes the Sovereign and Vigil conversations in Mass Effect so poignant is they embody the tone of a big, strange, mysterious universe that only the very original game managed to evoke. I don't need to profess my love for the Mass Effect trilogy any more than I already have, but neither sequel captures the galactic mystery and wonder of discover that the original does. The importance here is scope; Mass Effect does a phenomenal job of making you feel very small against the narrative backdrop. The illusion is convincing; despite design limitations the galaxy feels big. The sprawling, empty surfaces of uncharted worlds gentle illuminated under a vast, sparkling galactic sky is nicely contrasted by intimate conversations weighted with grand implications.
Sovereign doesn't reveal much and is really, in practice, an a-typical unexplainable monstrous villain taunting the protagonist. But to be faced with this, an enormous biomechanical terror with absolute confidence in a billion year long tyrannical grip over the galaxy, yourself a fleshy nothingness in its presence, narratively echoes the same tones as the aforementioned presentation and visual direction. Sovereign and the Reapers are very big things and you are very small and the scope and brevity of their importance and power dwarves everything else you've experienced thus far. Vigil is the historic thumbprint of similar tones; you stand in the ruins on a devastated civilization that was once the height of accomplishment and conquest, one that struggled in the greatest war the galaxy had ever seen, one they lost but still managed to leave a significant impact on despite the extinction of their entire species. And all that's left as a memory, that you are face to face with, is a broken, short circuiting AI glitching out history surrounded by lonely, moss overgrown ruins.
Mass Effect 2 and 3 have different highs and casually evoke similar themes at certain moments, but because of the tonal shift and pacing of the narrative take on different properties and tell their own story evocative of different themes. Andromeda had potential to accomplish similar tones to Mass Effect, but crumbled under an inconsistent narrative that is oddly overindulgent in making a strange, alien galaxy feel very safe, populated, and familiar.