Whichever one they tend to eat well and do well on. Sometimes it's hit and miss, but in most areas, you'll find the majority of showroom breeders use the same feed. That's a good starting point, though not always foolproof.
Another thing to consider--local feed mills. Often times, the local or regional feeds are widely used over national brands by showroom breeders. I likely would have never changed feed brands if Farmer's Pellet Company's "Oren Reynolds" rabbit pellets were available here. It's milled by a local feed mill in central IL, and is unavailable here. I used it to transition some rabbits I bought from up there, and found myself wishing I could just get it where I lived. The rabbits ALWAYS ate it well and thrived on it.
Stay away from feed with corn listed as an ingredient. It can cause digestive problems.