Published order, most definitely. Belgarath and Polgara both work better with a dose of nostalgia since they're not really prequels so much as recaps of the series from a different viewpoint with additional info. They are wonderful books on their own merits, but familiarity with the settings and characters will make them far better.

The Elenium and Tamuli series are also quite good. The Dreamers series was really, really bad. The stand-alone Redemption of Althalus was decent.