Oat straw is the most palatable if you can find it. A young healthy horse with good digestion and an ad lib supply isnt likely to colic on it, more important that the gut is working all the time on forage.
The important caveat to that is: best to have ad lib hay available as well as straw so that the horse is eating both, which lowers any risk of impaction as hay is thinner stalks.
Also, straw is very low in protein (3.4-3.7 against hay 8-13 depending when cut so, especially for a youngster, should not form the whole forage diet. It is also low in calcium. Hay has more protein/calcium, also alfalfa too.
With a greedy youngster who needs the nutrients, I would feed hay ad lib and something like D&H suregrow as a balancer. If you are feeding chaff that is generally chopped straw anyway, sometimes with alfalfa added, so would try for something else unpalatable as a bed.My very greedy cob doesnt eat her wood pellets/hemp mix