As mentioned, many choices. I am basically stillwater, but I do use sinking line on rivers because I fish still waters. I have all lines in 4, 5, & 6. Including the IV and V and to be honest, I have used those maybe twice in 10 years. II and III are used all the time with Intermediate in there.
I also have the Depth Charge line which is like tailingloop mentioned is 30' of a grain weighted sinking line (with 300 to 350 great for 6 wt) followed by 70 feet of an intermediate. This is a go to line in the summer or when fish have moved out deep. Kelly Galloups Streamer line is the same. And he knows how to fish rivers.
The other line of choice for me is the Type VII. It sinks like a brick and perfect for getting past those littler fish.
Also as tailingloop mentioned. Fly material makes a difference. Right down to how it is tied. Deer hair, tied tight has a cork effect, looser it will absorb water. Something to think about when fishing Muddlers.
I ended up making a Red Thread head on all my sinking flies so I could tell right away which are.