According to the very thorough introduction to David Thompson's "Thai Food" that would be pepper, in South East Asia at least. Pepper was also an important part of the spice trade.

Another spicy, uhm, spice that was native to the region (it's still used in Batak cuisine on the island of Sumatra) and to China is Sichuan pepper, which produces a strong tingling / numbing sensation when eaten. It doesn't keep very well, however, which might explain why it never made it to the rest of the world.

Root spices like horseradish and ginger can bring pungency, but not as much heat as chilis. As otherwise pointed out, black pepper and szechuan pepper have a bite. Still, old world cuisine simply wasn't as hot before the introduction of the new world chili. There's an excellent treatment on this in Krondl's "Taste of Conquest" which follows the chili backwards along the Portuguese spice trade into SE asia.