Unlimited, fresh, good quality hay is best, usually one with a variety of grasses, like baled hay, or hay from hayandstraw.co.uk or Timothyhay.co.uk. It costs more initially but if you compare the prices per kg of pet shop hay it actually works out lots cheaper
a 10kg bag lasts ages. The main thing is that they eat lots though, so if your bunny is eating a pile of hay equal to their body size everyday then you've found one they like!
My preferred pellets are Fibafirst, as they take longer to chew (better dental wear) and are the highest in crude fibre (30%, which is great for their tummies).
But there are lots of good pellet brands out there- for example Burgess excel and supreme science selective (Supreme are the same people that make fibafirst). Not all pellets are the same, so it's important to read the nutritional information on the packet - some foods that claim to be high fibre are only 14%! Avoid muesli- most rabbits don't eat all the pieces and leave the healthier bits, and most have added sugar and low fibre content.
I wouldn't feed ad lib to an adult rabbit, as it prevents them eating enough hay, which can then cause tummy and dental problems. It can also make them fat. The general rule is a tablespoon of pellets per kg of weight of the rabbit per day, which for the average rabbit is 2 tablespoons, roughly equal to a small eggcup. But a baby rabbit should be fed more, and some people do feed them ad lib.
Hope that helps :wave: