I'd say Nod32. I've had good experience with it and it's very fast with older computers since it's extremely light. I used to be a fan of Avast, but it's quite heavy these days. Both of them have pretty good detection rates.
I use Nod32 myself layered with Malwarebytes Pro both are extremely light on resources, have good detection rates, and are updated sometimes more than once a day. PC is right though Microsoft security essentials has come along way and isn't bad for a free solution.