The traditional BWF (using a Hereford) is always a good cross. But if you are looking for the best increase in weaning weights, you should use a continental breed. I (of course) would recommend a moderate framed (which most of them are now) Simmental bull. You could easily get a black one. Pick one with a Calving Ease of +10 and you can safely use him on your cows and heifers.
MARC research says that Angus is the #1 British & Simmental is the #1 Continental - so the cross is a natural. Simmental will increase milk production in your replacements, and improve Yield Grade in your carcasses, plus give you the increased growth.