The Riftwar Saga is by far my favourite series, purely because of the interactions between the main characters, of which Feist is very good at. The Serpentwar Saga is an awesome series as well, so I'd be looking forward to that!
Serpentwar saga was my fave, but thats probably due to the solid base that the Riftwar books provided. After those series then i enjoyed Honoured Enemy as a stand alone (it's set during the Riftwar), the Krondor trilogy were also quite good.
Despite owning two of them, I've never gotten round to the Mistress trilogy eventhough its HIGHLY recommended.
Empire, hands down, but that's largely because of Janny Wurts' involvement. Of the purely Feist series, it would probably have to be Riftwar, but I like different things about SerpentWar and Krondor, too, so catch me on a different day and my answer might change!
I would say that I enjoyed the Krondor Series the most, with a close second being the two books of the Legends of the Riftwar that I have read (Honored Enemy and Jimmy the Hand). They were all incredible page turners for me.
On a side note, has anyone read Murder in LaMut? I like that little legends trilogy, but that book didn't look very interesting to me. Like I said, Honored Enemy and Jimmy the Hand were great.
As a teenager it would have been the Riftwar by quite some margin, followed by the Empire series.
Now I find the Riftwar to be lacking in originality in terms of its world building in Midkemia (though it is still a very good series). Due the weaker middle book in the Riftwar series I would say the Empire series is just ahead slightly as a complete series.
Best series in my opinion is serpent war, was just superb from start to finish (tho i include kings buccaneer in that as its a precurser to what is coming), best stand alone book tho is honoured enemy that book is the best i have read in my life