The 270x will be the fastest and the 78780 won't be too far behind. The 750ti is the weakest out of the three and can not support sli.
If those are your options I would go with the 270x. You could crossfire the 270x though I would look into a more powerful single graphics card when you need to upgrade.
edit: didn't see he 760, that's the fastest card... but if that's in your budget I would consider a R9 280.