High Viscosity of Noble Gases
Viscosity in gasses is generally different than liquids.  Like liquids exhibiting fluidic properties, the viscosity is related to internal friction; however, the friction mechanism is different than inter-molecular interactions experienced by closely packed molecules. The molecules in a gas are far apart resulting in the influence of an entirely different friction mechanism.

The friction mechanism dominating gas viscosity is related to the number of collisions between gas particles. More collisions means more internal friction. In gasses, two dominant factors affect the number of collisions: (1) the mean distance a gas particle has to travel before it collides with another particle and (2) the average velocity of the particle within the gas.  

Some noble gasses (He, Ne, Ar) are monatomic particles with a lighter mass than the diatomic elemental gasses. The light mass results in a much higher velocity particles at a given temperature (consider  $E=\frac{3}{2}kT=\frac{1}{2}m \bar{v^2}$) resulting in more collisions. Xenon has a high molecular diameter, so the shear increase in size of the molecule increases the probability of a collision, thus reducing the mean distance it travels before a collision.          

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