Sengir's always interesting, though the whole problem with Homelands being such a lackluster set despite being a pretty fun setting always hurts. We know at least the Sengir bloodline and the Baron are out there undead and kicking, siring vampires all around the multiverse.
As for my favorite? I've always liked the idea that planes work on their own cosmological and metaphysical rules that are unique to planes. How Lorwyn and Shadowmoor are literally the same place, but controlled by the strangest Day/Night cycle ever. That Amonkhet is a plane that hungers for life so badly that it animates any and all corpses to slaughter the living and reset itself. That Mirrodin is actually pretty tiny and closer to a moon than a full planet. That Theros has not only a world that's stacked like a layer cake and ringed by rivers, but directly translates belief into things in the sky. That Kamigawa is a plane that exists half as a physical realm and the other half as a land of what are effectively sentient platonic ideals. All these interesting things that are part of the planes and give so many possibilities for things to happen on them.
It's like switching up the rulebook every time you go somewhere, and I love it.