What does the rival system do?
is there like a “Best rival” that u can get, and if so who?
rival system gives you rival
rival sometimes drops rare loot i think? from what i’ve seen you only get them for emotional attachment 
mine have always drop legendary and rare chart so yeah
the exotic chart:
Upon killing / bounty hunting same npc 3 times in a row you make them your rival, they wont be exectuted and will follow you from server to server.
Upon defeating them, instead of piece of their gear (like other huntabale npcs does) they will give you rare / exotic / legendary treasure chart