I voted for regular rats because they're easier to breed than mice (for most people) and grow to an appropriate size for large bps.

However, if you can get them asf's are supposed to be a very good choice for bp's. I've never dealt with them myself.

But I've also heard that bp's will get 'addicted' to asf's so if you went that route but then couldn't do it anymore you'd have a whole bunch of bp's that don't want to eat anything else. This would be esspecially bad if you ever decided to breed bp's cause anyone you sold a baby snake too would have a horrible time getting them to eat anything but asf's.