Discworld has been done, and is quite good.
The Lord of the Rings doesn't need a TV series, it has the movies and animated miniseries
A Game of Thrones isn't a series, it's the first book of A Song of Ice and Fire series and it's getting a TV series.
Wheel of Time is overrated, you'd have to take out 50% of the content after about book 5 for any TV series to be even mildly entertaining.
Have not read any of the Riftwar Saga (Magician) so I can't pass judgement there.
I think you'd have to choose a series that doesn't rely on mythical creatures that would be expensive to create and ultimately end up looking silly on a TV budget (such as Temeraire or really anything with Dragons).
Personally I think Kingdoms of Thorn and Bone by Greg Keyes would make for a great TV miniseries. It has its following, but it isn't so sacred that you couldn't change things to adapt to the new medium without upsetting a large fan community. It wouldn't be effects heavy, as the series uses mythical creatures sparingly. It may not be as high profile a source material as something like Wheel of Time, but as I mentioned that can be a good thing.