This poll is about the special encounters tied to the main quests of Oblivion and Skyrim.
I did not include Morrowind in the poll, since this poll only lists the encounters that affect the world itself during the course of the main quest. If Morrowind does have a world changing encounter, please tell me.

My answer is as follows (and will not include mods): I like the dragons the most, since they stay useful, even after the main quest, since they carry souls and large sums of money. They also seem more realistic in game compared to Oblivion gates, since the only thing that happens to gates is the occasional bunch of Daedra dancing around it. This while the game talks about a full scale invasion.
Oblivion gates only yield Sigil Stones, which get useless after the player gets access to the Arcane university for Enchanting and/or is using artifacts. Order Obelisks only contained useless loot, but did have an interesting way of closing them.