First of all, I think the people behind this forum should reconsider the colour codes of pie charts, because this:

is difficult to read to say the least. I assume it’s Dahl, Jakobs, Maliwan, Torgue and Vladof, but it could be a lot clearer with more distinct colours.
Now, first question. I chose Dahl (as many did, apparently) because there just is something satisfying about these military-style guns with high accuracy and extremely high bullet speed. In M0, I really like common Dahl snipers because they’re basically assault rifles on roids and headshot monsters. But of course they don’t do ■■■■on higher Mayhem levels, which is a shame. GBX seems to have completely forgotten about this style of weapon for high-performing legendaries.
Second question: I can’t answer this one because the most powerful weapons are whatever GBX decided to give a monstrous legendary. All common weapons don’t really play any role on M10. You have a Tediore (Anarchy), you have some COV (Yellowcake/NoPewPew), Vladof (Light Show, Plaggabur, Monarch) and so on that just annihilate anything that’s not in this group regardless of manufacturer.
Third question: I tend towards Dahl, because aside from Sand Hawk there’s not much going on there, and actually with its slow bullet speed it’s a very untypical Dahl weapon. Kaoson is ok in its current state but not exactly exciting imo. Given how I actually like their weapons if only they weren’t useless, I’d love to see more there. Atlas and Tediore are on the receiving end too, they deserve more love but I picked Dahl because I like their overall concept/feel much more.