Mono most guys use a #8 – #10 line. The venerable Trilene XT is a very popular mono for trolling applications.
If you’re looking to troll a braid I’d look at lines in the 4 – 6 Lb diameter. These lines will have 15 – 30 lb breaking test strengths and will cover you for walleyes in nearly any application.
My favorite trolling braid is spiderwire stealth. It is stronger and more durable than power pro or fireline and it costs less.
I used to use 15 lb Stealth, but it just didn’t seem to “cut it”.
Ask James, after I lost 2 of his “Finely Tuned” cranks on the rocks in a matter of minutes. Don’t know why the big difference between 15 and 20 lb, but once I switched over to 20 lb……… I never lost another crank to the rocks all Fall.