for me none of them, but my favorite is qihoo but....
1) if you are going to use qihoo, you must be online all the time in order to get the best cloud protection. why? well but i have seem on my testing that their bitdefender engine is not the same as the commercial version. I am saying this because for example i have a couple of samples that were not detected by bitdefender engine (talking about qihoo and of course it was offline) but THEY WERE ALREADY CATCHED in the commercial version of bitdefender (means BIS). of course these samples were 3 days old (actually bitdefender caught them a couple of hours since their first appearence). And for me as i am a student who is not always online and I HAVE TO use USB drives for everything i might be exposed to latest threats.
2) if your are not logged into an admin account you wont get protection (they know the issue, they said there would be a fix sooner, i am still waiting for it).
3) proactive defense consumes a lot of resources.
but for regular users is ok, no hassles settings, just install it and leave it. Also ads and crapware free!!!
COMODO:
1) you must be an experienced user, if not you will be lost.
2) yes, it blocks every program clean or not. (that is why you must be a pro user)
3) for every day usage is not recommended, for example, i am a programmer and some times i couldnt run my programs due to.
a) probably infected and sandboxed item.
4) i get lost with some settings
but, if you know how to use it, it may work just fine
PST: I love the firewall
AVAST
1) for my is very creepy that you are using youtube and suddenly an age restriction appear, AVAST pop up: you are not allowed to see this video, buy this XXX product and... Ok. Going to shop online... a pop up: try avast VPN and get protected.... Ok if you are using and AV to protect your computer from malware, why would you want that your AV is tracking your online activities.
2) I get lost with the settings. With this new version i have no idea where the settings for each thing is placed.
3) if you are a regular user who uses a computer for MS stuff and facebook of course you woul not have idea what is hardened mode, so with out this, avast detection rates sucks.
4) avast offers you a lot of crapware soft
but i like some features that for a free AV is awesome!!
For me
1) qihoo
2) comodo (if you are and experienced user)
3) avast