Not suprisingly this was the series I came to mention.You wouldn't be allowed to publish it now, but for me the first major fantasy series I read was
The Chronicles of Thomas Covenant
A friend of mine used that as part of the basis for his AD&D campaign. It was fantastic. I enjoyed reading the series afterwards too.My addition for the day: the World of the Alfar series (Isle of Skarpsey) by Elizabeth Boyer. Low fantasy, generally low magic, set in a Dark Ages pseudo-Iceland.