Great questions. Seems as if I find a brand I like, then it changes next buy.
Crossfire .095 string has been fairly decent.
Arnold .105 not bad.
Good old Oregon green .080 is my go to .080". Better than that MaxPower junk
I think freshness counts! Old hot dry stored string gets brittle.
I have a clam of Dewalt .080 trimmer line, which is better than average, but it breaks way easier than the Echo .095 Black Diamond. I also have some of the common green solid round line, which breaks even easier. No doubt, the breaking problem relates to my use, getting too close. Regardless, though, I think the "better" stuff works better.
And FWIW, with my old Dewalt electric, which cut well enough, when bumping the head for more line, many times the head couldn't spin the line fast enough for a clean shear on the trimming blade. The machine would hack and clack, until eventually the line would shear and get back to business. Never had this happen with a gas trimmer.