We use square gator line. We have tried stihl square line and some other brands of round line. We stick with the square gator because of durability. We don't use stick edgers so it is vital that our line lasts a while during use. I can usually weedeat and vert edge an entire walmart property on a full load of .095.
I use .95 Orange Nylium Oregon Starline after trying everything. If I get into some serious overgrown stuff I run the .99 Oregon Flexiblade and it just hammers through anything like it's butter. I was so impressed with the Oregon trimmer line I ordered an Oregon Pivotrim Pro head and have used it exclusively ever since. I use it with the precut 100 pack strips of orange .95 Nylium Starline.
i use .095 triangle or square. i have some rotary diamond cut (square) and i also just bought some rhino tuff triangle from home depot. 3lb spool is about $30. i never use round line because it doesn't cut as well.