I'm surprised so few have mentioned P Line CX. It doesn't last forever, but it's well worth it in my experience if you change it out each year.

It is very soft, low memory, ties good knots, with a small diameter to good strength when compared with many monos like UG.

If you like mono mainline, P Line CX 15# with 10# Ultragreen leaders has been a great combo for me for larger steelhead.

And I can't stress more, CXX is not useful to river fishing with lighter tackle. It will probably dissapoint. P Line CX is perfect and is an entirely different experience.