If you could visit one location in the ME universe, where would you go?
For me, it's easy: Ilos. I played through ME three times, and Ilos never once lost it's appeal. The awe and wonder I felt never faded, and today, hearing the "Vigil" track, visiting the Ilos page, reviewing the walkthrough pages which involve Ilos or visiting the Vigil page, all invoke massive feelings of nostalgia. For all the improvements of ME2 over ME, no place has captured similar feelings for me.
I think it'd like visiting Troy, but Troy is far more intact than it actually is, or visiting the Pyramids of Giza, but taken up to eleven because it's something far more ancient and intact. After all, the Prothean civilization thrived when proto-humans were scratching a living off rock.