When I used round coils I always liked Muji until I discovered Firebolt laces. They seemed to last longer and provide better flavour, plus the aglets made wicking quicker. Demon Slacker cotton laces are pretty good too. Never got around to trying Cotton Bacon.
Since switching fully over to mesh RTAs I've always used Vandy Vape M cotton as its nice and thick and doesn't collapse or sag under the mesh strip. Provides great flavour too. Firebolt have recently brought out their own M cotton too though so I've been using that lately as well and its really good.
Wotofo Xfiber is meant to be decent but I haven't tried that yet as when I ordered 2 boxes of the 6mm laces the shop sent me 3mm instead and I couldn't be arsed sending them back. If anyone wants them let me know and I'll pop them in the post. They're unopened and taking up space in my vape box so I'd be happy if they went to someone who'd use them.