When you are choosing a bit to start a young horse there are a few things to consider. A young horse will chew and champ on the bit, and perhaps at first rub its head to get rid of this new and annoying thing in its mouth. You'll want to choose a bit that is both as safe as possible and comfortable. Logically then a simple snaffle is the best choice. Leave any type of curb to more advanced training.

My choice would be a jointed snaffle bit with smallish rings that would be unlikely to catch on anything if the horse does try to rub its face. You'll also want to avoid spoon or full cheeked snaffles for the same reason. An eggbutt snaffle with a small rings would be my choice although I have used a loose ring.
Some people use mylar or similar synthetic mouth pieces (some are even flavored). But quite often these mouth pieces are thicker than a metal bit, and can be quite bulky in a young horse's mouth. I've seen lots of these bits with chew marks on them and I think a metal bit is less likely to encourage a horse to actually chew on the bit. If you want to add flavor to make biting a more pleasant experience its easy to smear on a bit of molasses, jam or honey. If you do choose a synthetic bit and a horse chews it, get rid of the bit before there is a chance the metal core is exposed.

Because small rings can be pulled through the horse's mouth, the first bit your horse wears may not be the first bit it is ridden in.

I think choosing a bit with a similar mouth piece makes the transition easier.

If you're starting your young horse in the winter time, make sure the bit is warm. A cold frosty bit is not a nice experience, even for a well trained horse. Make sure the bit is properly adjusted and fits the horse well. Too low or too high will cause discomfort and you want to encourage your horse to hold the bit quietly.