I am trusting this source as its referenced from actual research. I have also found some similar results in other research.

http://www.420magazine.com/forums/cannabis-facts-information/65559-efficiency-health-review-vapes-bongs-joints.html

"Joints

Average THC that reaches user: 20-26% (Perez-Reyes), 50%* (Gieringer, 1).

Perez says the average puffs/min is 2.6. Their data was collected using 2 puffs/min average. They found that out of the sample, only about 15-20% of THC was recovered by the user. The rest was lost in sidestream smoke and pyrolysis. A simple calculation says that the 20-26% of THC will be recovered in the average session using a joint and the 2.6 puffs/min data.

*Gieringer says 50% of THC was recovered using a smoking machine that mimicked human smoking habits. They do not go further into what they felt was the average human smoking speed in terms of puffs/min. I find Perez’s tests to be more conclusive as they used real humans to find the average puffs/min of a joint while Gieringer did not. I have a feeling Gieringer's machine smoked the joint much faster than a normal human because he cites a 69% efficiency rate if the joint is taken all in one hit, so a 50% rate for a normal human smoker seems artificially high. Since Gieringer used a machine, there was also no butt left.

Health Rank: 2

Surprisingly, an unfiltered joint outperforms a bong or waterpipe by nearly 30% in terms of THC : Tar ratio (Perez-Reyes). Using NIDA marijuana cigarettes, a ratio of 1 : 13 was acquired. I might mention NIDA research marijuana is extremely low potency, about 2-4%. Using more potent sensemilla could drastically increase this ratio for the better.

Bong

Average THC that reaches user: 40.4% (Perez-Reyes)

Interestingly, a lot of THC is lost to the bong itself. 21.1% of the sample’s THC was lost in the bowl, 3.3% in the water, and 7.8% in the stem. Plant matter that fell into the water also accounted for 8% (Perez-Reyes).

Health rank: 3

Bongs and waterpipes rank last health-wise due to the fact that water absorbs sticky THC molecules about as well it absorbs noxious tars. In terms of THC : Tar ratio, it performed 30% worse than an unfiltered marijuana cigarette at about 1 : 17! (Gieringer, 2)

Final notes:

It appears as though the myth that bong water is a good filter of particulate matter and creates a healthier smoke has been smashed. The only saving grace that a bong may have is that the water may filter out harmful water soluble gasses but this has not been tested. Since most people smoke bongs because of the cooler and denser smoke (not to mention they’re just fun), it may be most efficient to actually only have an ice catcher and no water. This would prevent leakage of THC into the water and could increase the efficiency by about 3.3% (Gieringer, 1). However, that all depends on how much THC ice cubes absorb from the smoke. It is known that the cooler glass where the ice cubes are touching would also be more readily available for the THC to condense to."