The absolute best in slot is probably some double rare MI combination that would be extremely difficult to farm normally. A more realistic best in slot would probably be something like:
Sandstorm Zarthuzellan’s Archive of Ruin
Overseer’s Skybreaker Circlet of Attack/of Readiness/of Ruin
Formidable Korvan Spaulders of Attack/of Readiness/of Ruin
Stonefaced Ugdenbog Girdle of Attack/of Readiness/of Ruin
Stonefaced Mark of Harvoul of Attack/of Readiness/of Ruin
As for what affix to choose over another, it sort of depends on what your build needs at the time. OA is very important for example, but if you have plenty of it already, you might want to look for more physical damage on the suffix instead. One of the benefits of using so many MIs is that the build is flexible and you can tailor it to meet your needs. It also means that there’s always room for improvement, and even I don’t have the perfect affixes yet (which is why the Zarthuzellan’s Archive has the “of Scorching” suffix instead of “of Ruin”, for example).
Regarding conversion, Skybreaker Circlet converts 100% Physical to Lightning on Vire’s Might and Zarthuzellan’s Archive converts 100% Physical to Fire on Vire’s Might. Together, that adds up to 200% total conversion, which is then multiplied by 0.5x to scale down to 100% total. So in the end, we end up with 50% Physical to Fire and 50% Physical to Lightning, none of which really matters since Vire’s Might only has 1 point in it and does very little damage anyways. Most of the damage in this build comes from Volcanic Stride, which is converted to Physical damage through global conversion on the dagger, chest, and shoulders, as well as Shifting Sands which is mostly Physical damage to begin with.