Hydrostatic weighing is the most accurate
they've been prooven to show a fair amount of inconsistency in alot of research.
Basically in a nut shell, they all work at giving you a "ball park" reading.
If you're training & making progress, measuring bf (by using the same method) will show whether progress is being obtained.
Honestly....my opinion.....just because you get some contraption telling you that you have a lower bf than the guy standing next to you....it doesn't insure that you look better than him.
It's a tool....just like anything else.