Ha, a loaded question, but I'll echo what others have said: 

A dream RP would check the following boxes:
-A strong cast of main characters that are flawed, complicated and lovable
-A fleshed out world that borders on Tolkien levels of lore 
-Defined arcs that explore different aspects of the main characters and/or lore. Each arc could alternate between earth-shattering, "AAHH the world is on fire" and intimate, more grounded relational moments. 
-Funnily enough, a defined end point. I don't know if I'm weird for wanting this, but every story, no matter how amazing, no matter how riveting, has an end. I'd love to assemble all this and play this out over the course of a few years, but I want a defined end point to work to so that the stories we've made don't ever get stale, that we'll remember each moment fondly, but not feel obligated to keep churning out plot for the sake of plot...if that makes sense?