I find it that the favourite at any one time varies an awful lot between batches. Most of the bunnies here will happily dive into whichever hay they are given at the moment, I tend to just mix it up a bit to add variety and keep them stimulated. The Ings hay from Hay for pets is always a firm favourite; over the years I would say that oat hay has been the most universal hit, even persuading non-hay enthusiasts to have a munch.