I don't think I could pick just one, but I can tell you some I am considering. Of course, when picking my favorite one, I have to omit bulls that I have owned (Big Jake and Lazy D Ultimate Type were the two best breeding bulls I've had in 50 years).
Deerpark Leader 18th is a bull that I absolutely love for productive cows. Every one I have been associated with was moderate, good uddered, and fertile. I think JIT would say much the same about his 18th son, Irish Mist. My Leader 18th calves have come easily, too. I'm using a Leader 9th son on heifers and have had good luck, so far. More to come this year. I am planning a mating of his daughters to Leader 18th for the next generation calving ease bull. I've always admired the Trump X Nobody's Fool line crossed with Leader 18th. If I had unlimited funds and time, I'd flush all my Nobody's Fool bred females to Leader 18th. The Leader 9th son's calves just don't have enough performance, but he works for heifers and that is his purpose. I know there are supposed to be some other bulls out there for calving ease that would perhaps have more performance, but I've had experience with this one and trust him.
As far as the main herd, I've got a mating in mind involving some fairly retro genetics, not quite back to the 60's and early 70's, but a popular bull that was extremely correct, heavy muscled, and moderate. He was considered by many to be too moderate in his time. I'm not ready to say what it is. I'll start my experiment this year and see what happens.