King's Quest III and IV are my favorite in the KQ series. But overall I'd probably say the first Quest For Glory. It was such a great melding of Sierra adventure and RPG.
Couldn't agree more! I got into QFG1 at the time of Sierra's VGA point-and-click remake, and what a lasting impression.
The artwork, music, sense of humor, and character building - including being able to import your character and keep building your skill points through each successive game in the series - made it really special. Truly the golden years of graphical RPGs, IMO.