Well theoretically Hydrogen (H2) would probably be the best.  Here is a list
of different gasses and their molecular speeds to see for yourself...

Gas                                                        Vrms  (m/s)
Hydrogen (H2)                                        1920
Helium (He)                                             1370
Water Vapor (H2O(g))                             645
Nitrogen (N2)                                           517
Oxygen (O2)                                            483
Carbon Dioxide (CO2)                              412
Sulfur Dioxide (SO2)                                342

(all speeds are at room temperature, 300K)

Simple formula to calculate the speed is this...  Vrms = ((Sq Rt) 2RT/M)...
where R = (gas constant) 8.31 J/mol*K)
           T = Temperature (in Kelvins, K)
           M = Molecular Mass
ok that's enough chemistry, time to get drunk :-P

-Shant