Straight-grain and hand-split.
Hickory or Ash are probably most common, but if you can find it then either Osage Orange or Pecan are both strong and moderately flexible even when fully cured.
I know a few people that make bows from Osage and it's strong, flexible and very nice to look at too. The pecan I have for walking sticks has retained flexibility and can support quite a bit of weight too, wears like iron, but is difficult to find a straight-grained section long enough for rods.
vic