So you could justify a poor Steven Segal movie as being better than a good Arnie movie could you?
I see you're still struggling - and you're still using these words because your brain can't deal with not using them. There's no such thing as a poor Steven Seagal movie. There's a movie you might consider poor - but that's different.
So, I can't justify anything relating to your opinions - that's something you have to do yourself.
(assuming you can cope with the concept of 'personal opinion' being a different concept to 'public opinion'…)
The challenge for you, here, is to realise that your perception of public opinion is not necessarily the only perception.
If you want to establish why this can't represent public opinion - you have to provide proof - not speculation or anecdotal "evidence" conveniently altered to suit your agenda.
Ultimately, it's an opinion piece - as you say - which makes everything about it as correct or incorrect as any other opinion piece.
As for its objective value - it's no more or less useful than what you imagine would represent public opinion.
Certainly, I have a very easy time imagining that my opinion of DA2 does NOT represent the public opinion. It has enough superficial values to be considered a good game in the public eye - as it tends to represent casual gamers more so than core gamers.
As for Skyrim - it's just about the best example of a "best game" as I can think of - because it can be appreciated greatly by both core and casual gamers - and it can be tweaked every which way through the modding community.
I consider such a game much more likely to be the "best" in the public eye than whatever we here at the Watch would prefer.
Even so, that would be nothing but my perception of the public opinion and - as such - it would hold no value as an objective observation.
Edit: In which case why not have a list which puts ET original release as the number 1 computer game of all time… it's just one person's 'opinion' - but what 'credibility' should such an 'opinion' have?
I don't know what value you assign to opinions. That's another subjective consideration.
To me, an opinion by itself is only as useful as the context within which it's provided - and the insight with which it's supported.