What is the fastest horse in Mount and Blade Warband?

The Courser is the fastest horse type in the game, with a base speed stat of 50. A horse with the Champion variant grants +4 speed, and a horse with the Spirited variant grants +2 speed.

This means that a Champion Courser combines for a blistering 54 speed, whereas the standard Warhorse has a speed of 40.

Source

As said already, Courser is technically the fastest, but using a Courser is very risky due to very low HP, especially if you face the nords with their throwing axes -- one hit in the head at its full speed, and you are not on horseback anymore, and your Courser is likely no more Champion (crippled). So you should probably go for increasing Riding skill if you really want to be fast. If you want more speed than the armoured horse (Charger, War Horse), you should probably try Hunter -- it's relatively fast compared to armoured horse and also relatively tough compared to Courser.