I cannot but agree with 'teeteringhead', it depends entirely on what you mean by 'successful'. Sure, the B206 is (today) the eponymous helicopter, but what about the B47? From today's perspective it is difficult to realise that, 50 years ago, any other helicopter existed!
On a purely personal level, I would have to vote for the Westland Wasp/Scout on the grounds that, whilst not a world-beater, it did exactly what it said on the tin, and did it bloody well. In its latter years it also did a great deal that was not on the tin, but that's an entirely different issue!