Cedar is a classic boat building wood, but is used mostly for planking. It would be the lightest and probably the easiest to bend. Oak is used more for framing in boats and would be the strongest, but also the heaviest and hardest to bend unless it was steamed. The late Robb White preferred to use poplar for his boats. Ash would be a great choice if available. That said, I just used some clear pine for the gunnels of my pirogue. Home Depot here had it available in 16 foot lengths.