Tali.
Now, this is one of the few Tali posts that doesn't consist of OMG DO WANT QUARIAN SEXYTIMES. Although there's a little bit of that in there, not gonna lie. No, the main reason I want Tali?
She's a starship engineer with a boatload of medical tech.
Think of what this means!
If she comes armed, we can get our hands on element zero. And, since humans are innovative creatures, we will be able to replicate it. And -then- we've got a real stellar expansion drive knocking about. We can claim the stars around Earth without having to worry about crazy-ass turians getting pissed off because of it. We can expand without the reapers. We can become the aliens from Fifties films because we don't have to fear the universe any more. We can leap into the unknown and build a civilisation. We can do it.
Because, despite fantasy being awesome, real humans have the ability to be more awesome than any fantasy can ever be.
Let's face it, guys. Humans rock.