F1's can be any number of combinations resulting in a first generation offspring from the mating of two full blooded bovines, such as the first generation offspring of a mating of Hereford & Angus (the result in my neck of the woods is called a "black baldy"); or a Gelbvieh/Angus mating (I think some folks refer to these as "Balancers"); or Hereford X Brahman.
I don't know about other parts of the country but, from a "commercial herd" standpoint, along the Gulf Coast region of Texas probably the most desired cow (based on the action at the spring and fall replacement sales) is the "tiger stripe", with black brangus a close second. The "tiger stripe" is just the common vernacular for a cow that is 50% Hereford and 50% Brahman, most typically the result of breeding a Brahman cow to a Hereford bull. These crosses attain good size, have good mothering abilities and can hack it quite well in our high heat, high humidity, high parasite environment. The hottest thing going in these parts seems to be using a Charolais bull on tiger stripe cows for a terminal cross. The resulting calves, in my experience, always seem to sell at the top of the market. Using a black Angus or black Brangus bull on tiger stripe cows also works well.