What is the fastest weapon to run with?

A knife or a bomb. Unless you wish to shoot, then a Tec 9 or Five Seven

Terrorists

Knife - 250 Velocity
Bomb - 250 Velocity
Grenades - 245 Velocity
Tec 9 - 245 Velocity
Glock - 240 Velocity
PP Bizon - 240 Velocity
Desert Eagle - 230 Velocity
UMP - 230 Velocity
P90 - 230 Velocity
Mac 10 - 225 Velocity
SG556 - 220 Velocity
MP7 - 220 Velocity
Scout - 220 Velocity
Nova - 220 Velocity
AK47 - 215 Velocity
AWP - 200 Velocity
M249 - 195 Velocity

Counter Terrorists

Knife - 250 Velocity
Grenades - 245 Velocity
Five Seven - 245 Velocity
P2000 - 240 Velocity
Glock - 240 Velocity
PP Bizon - 240 Velocity
MP9 - 240 Velocity
Desert Eagle - 230 Velocity
UMP - 230 Velocity
P90 - 230 Velocity
M4A1 - 225 Velocity
Mac 10 - 225 Velocity
AUG - 220 Velocity
MP9 - 220 Velocity
Scout - 220 Velocity
Nova - 220 Velocity
AWP - 200 Velocity
M249 - 195 Velocity

From here