/sci/ - Science & Math

I'd say fusion power.

A space elevator would require a global worldwide-effort. Humanity is too selfish to focus on anything other than money. There's probably no benefits (yet) that would justify mobilizing that much ressources.
Also, I think that nothing big can be done in space until we find a way to send humans and materials cheaply, rapidly and at a large scale, without any physical link to the Earth. I guess the most realistic way is building ships strong and reliable enough to be able to enter/exit atmosphere using its own propulsion methods.

But yet again, that would require some serious engine technology, aerodynamics constraints and an energy supply that doesn't exist yet.