First Party Campaign Worlds
1) Eberron: I admire how the world is so internally consistent despite the myriad of things going on in the world. There so many ideas to plunder for tinkering with my own homebrew settings. Eberron is also one of the few settings that integrates magic into the world's historical development and industry. Dragonmarks and Dragonmarked Houses are also ingenious.
2) Dark Sun: Post-apocalyptic sword & sorcery. Brilliant. The world is harsh and alien. The world hates you. Life is nasty, brutish, and short. Arcane magic drained the world of life. There are no gods. All the racial archetypes are turned on their heads and taken to extremes. Dragons are a process.
Third Party Campaign Worlds
1) Diamond Throne: Much like with Eberron, I am so thoroughly amazed at how integrated the magic of the Arcana Evolved is with the setting. Magic rituals are an integral component of the various cultures. Giants use rituals to grow larger, become more peaceful or warlike. Rituals were used to transform into the mojh race and create the sibeccai race. Runes possess an almost mystical life unto themselves. People use minor rune manifests as boons to accomplish everyday tasks in their life. Major rune manifests embody concepts such death, war, and disease. There are even runechildren and rune messiahs. Truenames matter. Swearing oaths matter. I also love the purposeful departure from Tolkienesque fantasy races in favor of fantasy more akin to The Land in Thomas Covenant or Earthsea.