To my mind, there's only one king of the isometric RPGs, and that's Neverwinter Nights. Not only did it offer up an amazing campaign that gave you the chance to save the city of Neverwinter, it also incorporated something that broke new ground: a robust multiplayer mode with support for custom content. The Aurora toolset allowed players to create their own adventures, and they did so with gusto. By the end of 2002, there were already over 1,000 custom adventures for the game (It released on June 18, 2002). Add in the three official expansions, and you're got a game with a mountain of content that's super fun to play.
You can check it out on GOG.com.