[Ford Prefect]

I really like some of SM Stirling's stuff. In particular the Island In The Sea Of Time series, the Dies The Fire series, and The Sunrise Lands series ... all three series inter-relate. Most of his stories are alternate history/alternate reality stuff, including the three multi-book series above. I think he's a lot better than Turtledove, who seems to be considered the top dog of this genre.

Conquistador is a good single book tale if you want to just sample his stuff.

I did not enjoy Stirling's Peshawar Lancers or The Sky People.

I have enjoyed David Eddings, but he's more fantasy than SF. Same with Stephen R Donaldson.

Another fantasy writer I really like is Canada's (and formerly Calgary's) Jack Whyte. He has an Arthurian series that's wicked good, and he's currently working on a good Templar Knights series. He's categorized as fantasy at least, although I find his stuff to be more historical fiction than fantasy. He's good though regardless of how you want to classify him.