Depends on the bait and cover.

For a Lucky Craft BDS 3 or RC 2.5 I like 15-17 florocarbon. I throw them on a 7m rod.

But for smaller wood baits like a Dbait I use 12lb copolymer or 15copolymer if its heavy cover. I throw them on light 6'6m rod.

There isn't a best combo and line for every squarebill in every situation. Fishing rip rap and fishing thru heavy brush are much different.