I've been through many barands for long periods of time. My biggest suggestion is, get the lightest you can handle playing metal on. They are more toneful that way. The lower the tuning, the fatter the strings. I've gone up to a 120 on the E string for tuning down to "C". No tone at all.
For the past 6 years I've been using D'Addario XL165. That's the medium gauge set. I also put steel string on some of my instruments. I love Rotosound but they don't make a proper 6-string set which is what I play normally. I have recently discovered D'Addario Pro Steels, just like Rotosound but the tension feels a bit more even. Everyone has been commenting on my tone since I put these on.
OTHER BRANDS: I used Ernie Ball for 4 years. After going to D'Addario as an emergency, I realized the string tension on the Ernie Ball's weren't as tight as other brands, even when the sizes were the same. Ken Smiths are cool, the Rock Master series, hard to come by and even harder in specific gauges. I've also tried the D'Addario Prisms, they were nice but the most expensive 4-string set I've ever gotten next to Thomastic Infled.
I would never suggest flats unless you are Steve Harris or playing Motown or Staxx. Best of Luck.
