I'd say the NQ, because its the smallest leverage. NQ is worth about 34K$, as opposed to YM (55K$). However I've seen the NQ explode one way or another, whereas the YM sometimes holds up better when the market tanks.
i used to think the ES was the best to start, but not anymore. I actually think now the best is the midcap 400. Good for small volume traders, no program trading so the trading tends to be more real.