I've tested several since the time of Windows Vista. For me the ones that have evolved the most in terms of Real-time Protection, Performance, Malware Detection and online protection fee, would be:
1. Avira
2. Bitdefender
3. ESET
4. F-Secure
5. K7
Avira I have used for a long time and currently, for me it is the best anti-virus on the market.