Another vote for Avast free - definitely, without a doubt, it is the best current free AV product bar none. I have it combined with the paid version of Malwarebytes and they make a very effective combination. My distant second choice for a free product in a pinch would probably be Avira.
If I were paying for AV, NOD32 is a very solid choice. That being said, though, I'd also have to seriously consider trying the paid version of Avast. They have put so much effort into improving the free version over the last few years that I would find it hard not to reward that effort by making a paid purchase.
MSE is light on resources, easy to use, and pretty effective against viruses. However, it also has a very significant downside: it is pathetically weak at preventing malware infections from drive-by downloads. I won't use it any more myself or recommend it to others until MS takes steps to address this issue. Combining MSE with the paid version of either Malwarebytes or SuperAntiSpyware would probably be a good interim step.