Across the board answer...? HEAVY disc. If conditions allow (ie roots cleared/gone) a tiller might work. Plow or chisel plow ONLY if roots aren't that much of an issue. As stated, the goal is to incorporate mulch into the upper part of the soil structure, and NOT about deep tilling. V ripper/subsoiler/chisel plow, ect can be an exersize in futility if roots still present. Heavy disc is the safest/best choice if conditions aren't absolutely perfect. The big "excavating contractor" style disc pictured in Egons post is a good example....IF.....you have enough tractor to pull it. Many contractors who do land clearing will have a disc like that. Those big heavy ones
cut up small roots and handle the occasional big rock better than a tiller. Under MOST conditions, a lighter disc, possibly a tiller, or some other sort of fiinal tillage will still be needed.