[Ruatha]

Asimovs Foundation series is an all time high.
But I also like Jack L Chalkers Well of souls series, quite original:
Well World
(Midnight at the Well of Souls. 1977)
(Exiles at the Well of Souls. 1978)
(Quest for the Well of Souls. 1978)
(The Return of Nathan Brazil. 1980)
(Twilight at the Well of Souls. 1980)
Watchers at the Well
(Echoes of the Well of Souls. 1993)
(Shadow of the Well of Souls. 1994)
(Gods of the Well of Souls. 1994)

Also Ursula K LeGuin wrote good SF and Fantasy.