I see avg free, MSE, and avast all work fine with W7. I recommend MSE to less experienced users, I personally use avg right now, but It's hard to say which is "better" unless you want to compare some AV comparatives or something.
ive seen there isnt one that is "best", but there are alot of good options. Avast, MSE, avg, and vipre ive used are all great, i especially recommend avast and vipre. ive also heard avira and trend micro are good but havent tried them myself.