I don't like to injure the fish by breaking off etc.
So for me it is 20#.
Though I admit I usually use baitcasting gear.
I don't have trouble with long casts on 20# line.
usually it is Berkley big game line.
Years ago I was all mixed up with some IGFA record chasers from Florida. They came up to look for light line silver salmon records, opting for lake fishing over rivers so they didn't have to contend with currents. Saw a lot of big silvers caught on 2#, 4#, 8# and 12#. Due to the lack of current and open water they were able to do it with all of them. The 2# took a longlonglonglonglong time, and the 4# was only a little better. It could be done, but none less than about 20 minutes. Things were better on 8# at around 5 minutes, and a couple or three minutes on 12#. Add current and all those times would stretch a bunch.
If you plan to release any fish, due to being too red, too small, or just wanting to fish more, don't go too light. If you don't wish to break off a lot of snags or fish, don't go too light. If there are other people fishing around you, don't go too light. 15# mono is about as light as I'd recommend. I've used lighter myself, but it depends largely on the stream, the tackle I wish to use, and if I plan to release anything.
My two spinning rods each have Mono, one with 17# and the lighter setup with 8#...caught fresh silvers on Susitna and in Gastineau Channel. I drag my fingers over the first four feet every ten casts or so and after snags to check for knots and fraying.
I use 14-17# mono or 30# braid on spinning reels. I carry matched 9' rods/series 40 reels, one with mono and the other with braid. If the fish is hooked in the mouth the mono is plenty strong...snagged is iffy.
That's what a I use down here.....usually tossing pink marabou or bunny jigs on 10# It's jus a decent daiwaspinning reel on a 7 foot walleye rod basically. With this rig I can fish behind the spinner chuckers and work water they could never reach quite right.