I'd have to say smoking a joint is probably one of the least healthiest ways to consume cannabis. Remember, you're not just breathing in cannabis smoke with a joint, you're also smoking the paper as well. A waterpipe is most likely the healthiest after vaporization, but remember that waters filters out THC as well as tar so you'll probably be smoking more to get the same effect which could negate the benefits. There's always hot-kniving though.
Honestly, I suggest just using a pipe or a dry bong.
"'Everything is true', he said. 'Everything anybody has ever thought.'" - from Do Androids Dream of Electric Sheep? by Philip K. Dick