In order to find the best deal, let's divide the cost by the number of pairs of socks, this way we can find the cost per pair of socks.
For the first deal, we have:

So the cost of each pair is $100.
For the second deal, we have:

So the cost of each pair is $120.
Since the cost per pair of socks in the first deal is smaller, the deal #1 (6 pairs for $600) is the best deal.