The Egg Bag (popularized by Max Malini).
This effect is very simple, very impressive, and allows for many creative interpretations. It is pure magic (something vanishes entiretly, then reappears, and the audience's suspicions are proven to be unfounded in the process). Additionally, it provides a wonderful opportunity for audience participation, which allows the effect to be repeated in the spectator's own hands. Even he/she will not see through the secret, making it all the more incredible. The effect truly is among the greatest stage effects, in my opinion, due to the simplicity, purity, interactive nature, and bullet-proof design.
Jeff Hobson's is the best Egg Bag on the market, to my knowledge. You may want to get a second opinion though, if you wish to persue this idea.