The Boomerang was designed and flow in such a short time, it wasn't up to standard in the period that it was released. However, remember by the time it was ready and in production Australia had P-40's and Spitfires available and they occupied the fighter role. I think I would be correct in saying it saw very limited service as a fighter. As has been said earlier, it was a very effective close support plane.
Whilst as a fighter in was inadequate, I don't think it saw enough frontline service as a fighter to be considered the least successful, how about something that had a length of time to prove itself, yet still failed badly.
The Defiant got massacred as a day fighter, but was a fairly successful NF.