There is no "Best Antivirus". It doesn't exist. Unless every single Antivirus left the industry and was shut down except ONE, there will be no "Best Antivirus".
Each Antivirus product has different strengths. For example, I personally feel that ESET has a good detection rate with their signatures/heuristics compared to the other products. Their weakness may be the default settings for the HIPS, or the HIPS in general. Another strength they have is a good web filter, and performance on the system whilst ESET is protecting the user. Most users don't feel any system impact at all.
Emsisoft have a well-loved Behaviour Blocker zero-day protection component working alongside their cloud protection to verify suspicions the BB may have, also providing more features such as auto-decisions based on what other users did with a detection (if the cloud is enabled of course). Their weakness could be the signature-based detection/PUP + PUA detection. They may have Bitdefender, but that's not to say their own engine could be improved. Emsisoft also have a good reputation for a very clean user interface - everyone has their own opinion and like other vendors, however I also feel that way about the Emsisoft UI. However, at the same time, Emsisoft could try to improve the system performance whilst running the product - I have been a user of both EAM and EIS in the past, and I did experience a slowdown. It was not that bad to remove the product, however I noticed it.
avast! have a feature called DeepScreen. This feature will help detect malware before allowing it to execute malicious actions on the users system (gets uploaded and executed in the cloud whilst being virtualized). This will allow avast! to log the behaviour occuring during execution and decide if it should be allowed to run on the users system or not. This also prevents many malicious software from executing on the system.
Kaspersky have the Application Control as part of their zero day prevention protection, however some users experience some issues with this feature. Kaspersky could get this fixed to improve their product. Another weakness to Kaspersky could be it being heavy on the system.
Qihoo have a sandbox which people may appreciate and want to use. They could improve on their false positive detection (removing the false positives).
Panda could maybe think about how they work their updates - not that long ago they had experienced issues which left some users unable to boot Windows. Panda did address this issue and do everything they could to get their users back up, however it still happened.
Norton have their reputation-based detection which does help detect a lot of potentially suspicious programs. This could be a strength for Norton.
At the end of the day, each user has their favourite and it's up to the user, no matter what advice is given. Advice is given to help give the user a understanding towards the product (and maybe based on their own experience). It's down to you to decide what product to use. I recommend you go through using some trials (most products allow a 30-day trial) before purchasing one to find the AV you like best.
Every vendor has somewhere they can improve on. There is no "Best Antivirus".
Cheers.