I use whatever the chart says. There are charts out there that recommend x buffer for y AR and they differentiate by barrel length, caliber and length of gas system. If I need to I'll tweak it from there but I've never had to.
Usually some kind of H buffer, maybe an H2. Don't think I have anything heavier. I keep Wolff springs around for replacements but can't say as I've ever actually had to swap a spring unless it was shot.
My secret to reliability is the use of the JP Ent. full mass carrier, slick and they WORK. It's all I use now. Costly but SO worth it. Everything else is mostly milspec/KAC type stuff with a good barrel.