Meramac River probably. Where have you fished?
My favorite is the Elk River because of the combination of numbers of fish and the chance for some really good fish. It has a very good mix of deeper holes with downed timber to shallow fast water spots, bridge pilings, boulders, and roots from trees on the bank. I've never flyfished for them but I'm sure you could catch them doing it. I haven't fished the Meramac but I've heard it's a really good one and would like to fish it sometime.
I've never heard of the Elk river. I'll have to look into that. This will be my first time fishing the Meramac for Smallmouth Bass so it'll be an adventure.