No one breed is best. The availability of a breed in your area. The amount of money you are willing to spend to purchase your team. The local demand for particular breeds at resale time. The temperaments, size, and growth of each breed.
All breeds have weak and strong points.
When you find calves you think will work for you, don't get in a hurry to buy them. Of course, if you're looking at heifers they should appear feminine. As you look at potential calves, try to envision the ideal ox in your mind and relate that to the calves you see. Select animals that will develop and grow similarly so that they will look and work alike as a mature team. No one can guarantee that the calves you purchase will come together and work as you want them to, but these guidelines will help you in selecting ideal calves to train.