RE: If you could have one:Disk Harrow, cultivator, moldboard plow???
2 questions...
1) What do you intend to plant
2) What do you plan to pull the implement with??
My first response would be a disc, as with a plow you will not have a smooth seed bed, plows are more for controling weeds and insects and if you plow in the late fall in northern climates and have clay the winter freezes will help break up the clay....You can also use a disc to incorporate lime, fertilizer and plant crops like wheat, rye, oats, etc...