I guess this depends on how big the stops are you want to have.
If you want to learn the ES, then the best proxy to trade is the S&P. .
That in my view is absolutely correct. The only reason I haven't used the 500 stocks of the S&P was simply because I wasn't ready to pay Esignal the costs for 500 symbols. However, as luck would have it, a member of Esignal has been following my threads on ET and has kindly offered me a deal on all 500 symbols. Therefore, I will be using S&P as my proxy in the future...