The Century Optics 0.7 wide angle converter is the best you can get. If you are on a small budget you might consider the 0.6 wide angle adapter from Century but it has many disadvantages...e.g. it's not zoom through,...
Pana makes a 0.8 wide angle as far as I know...it's said to be ok, but I only use Century and Schneider glass in front of the superb Leica lens and have never had any problems. They are high quality and worth every penny...