Ask 10 people and you'll get 5+ different answers. A nice sounding exhaust is 100% in the ears of the listener. I don't like loud and obnoxious sounding trucks so I like my 18" Maggie. The next guy might like the sound of a muffler delete and so on a so fourth.
I would say the most popular mufflers on Rams seem to be Magnaflow 14/18", Cherrybomb extreme, flowmaster 50.
Go on YouTube and start listening to different Ram exhausts and figure what YOU like, not someone else.