I've seen all three shows extensively and actually prefer the BBC show the most. IMHO, the BBC one is the most serious version and even though you have to get your head around some of the UK-specific stuff at first, you'll learn a lot from the show. Canada's Dragons Den and, even worse, Shark Tank, are more heavy on the drama and personalities, so there's more flash, but less substance.

Also, Peter Jones on BBC Dragon's Den strikes me as an incredibly intelligent investor.