Liquidity is the only measure that really matters for money. One measure of liquidity/market depth could be: for how many USD can you buy/sell while only moving the price, say, 1%. To get a more detailed picture you could plot it on a graph with USD amount on one axis and slippage (price change) on the other axis.

You'd have to merge the order books of all exchanges for a particular coin, and execute the order against that. I'm fairly certain that by this measure, Bitcoin is multiple orders of magnitudes more liquid than all other crypto currencies combined.