I agree it would probably be litecoin, but disagree on the reason. Market cap is a meaningless concept when applied to cryptocurrencies. It's calculated by multiplying the number of coins in existence by the price per coin at the time of the last sale. A little more meaningul would be trade volume in a fixed time peried, measured in common units, e.g. BTC or USD. However even that isn't very meaningful because some trade volume involves mixing or moving coins already held.

Litecoin seems to me to have the most established market among the altcoins at this time, but its standing might change in the future.