The most feasible hypothesis about Theia The formation of the Moon had many hypothesis over the years, most of them involving a Mars-sized body called Theia, which presumably collided with the Earth. But even within the Theia hypothesis, is not very clear the conditions of that collision. So, what is the most plausible hypothesis about Theia today? 


Theia fully disintegrates and the remnants debris form the Moon in a head-on collision(the Big Impact hypothesis)
Theia-Earth suffer a glazing collision and Theia manages to escape to space, but some of the remnants of the collision form the Moon (Reufer, 2012)
Other hypothesis, such a double Theia system


All the hypothesis have problems, but to me the most believable is 2, because it solves the majority of the problems of the other ones, although there's not much consensus on that one, the only who defends it is Reufer.

For example, some research shows that only 2% of Theia actually contributed to the formation of the Moon (I don't remember the reference), so 1 doesn't seem too much feasible because according to that one Theia fully disintegrates.
According to new simulations, a better model seems to be one where the Earth had a magma ocean at the time of impact, see here:

We perform density-independent smoothed particle hydrodynamic collision simulations with an equation of state appropriate for molten silicates. These calculations demonstrate that, because of the large difference in shock heating between silicate melts and solids (rocks), a substantial fraction of the ejected, Moon-forming material is derived from the magma ocean, even in a highly oblique collision. We show that this model reconciles the compositional similarities and differences between the Moon and Earth while satisfying the angular momentum constraint.