At the time of writing, 3Dception and AstoundSound would be my top picks in terms of sound quality, performance, stability, and ease of use. 3Dception feels like the most mature plugin with lots of advanced features, and AstoundSound has the advantage of working over additional hardware setups. Plus I think that both of these plugins sound great.
The other plugins in the list all have a lot of potential, so I would recommend keeping an eye on them.
Oculus Audio SDK is free to use, but has some work to do in terms of sound quality, user interface, and stability.
Phonon 3D needs to address performance, and then continue to incorporate their three plugins so that they can be used in tandem.
RealSpace 3D has the foundation of great research, but it has work to do in terms of performance and usability.
See the full technical post here: http://designingsound.org/2015/04/3d-audio-weighing-the-options/