As far as I know, the most successful system trading on currencies are channel break-outs. Not sure about stocks, but similar systems will probably perform the best due to the small risks of them. And it has a straight forward investor psychology explanation: volatility levels off, investors getting bored, tight channels form... then price break out of channel, bring more and more interest from public to give the price higher lift.
Other technical systems such as moving average crossing, stochastics, RSI, macd etc may only work marginally, if they do at all.. since they all have much bigger maximum drawdowns. and it's hard to give the signals a straight forward psychology relevance. (you see RSI rise from below 30 to above 30, but what does it mean to the investing public? It's too fuzzy. How do you distinguish this from RSI rising from below 29 to above 29? you see, it's too artificial)