Anti-aliasing is a technique to get rid of jagged edges of a VR scene which appear as a result of square pixels forming non-rectangular shapes.
Fast(2X) : It's the least demanding form and smooths out edges with a minimal performance hit but it also blurs out textures. Makes them look a little muddy.
Beautiful(4X) : It makes the image look much more natural in nature and the images are crisp.
Fantastic(8X) : Gives a big performance hit but does the smoothing out job much better than 4X.
InstaVR recommends to use "Anti Aliasing" as "Beautiful(4X)".