What is more important to success, the cards you play (best deck), or the cards your opponent plays (metagame)?
This question depends on two things, the format and the metagame.
Yes, the answer to the question "which is more important, best deck or metagame?" depends on the metagame itself.
But first let's look at how the format relates to this question.
Suppose we have a hypothetical format called "Type Academy". In this format there is a deck called Academy, and a number of other viable decks. The win percentages of Type Academy are:
Academy beats everything else 90% of the time
Everything else beats everything else 50% of the time
In this format the answer to the question is clearly "playing the best deck is most important to your success".
Now let's suppose another hypothetical format called "Type RPS". In this format there are three viable decks: Rock, Paper, and Scissors. The win percentages for Type RPS are:
Rock beats Scissors 90% of the time
Scissors beats Paper 90% of the time
Paper beats Rock 90% of the time
In this format the answer to the question is "considering the metagame is most important to your success".
Finally, let's suppose a third hypothetical format called "Type Balanced". In this format, there are dozens of viable decks. The win percentages of Type Balanced are:
Everything beats everything else 50% of the time
In this format the question is completely irrelevant, success is purely determined by luck and skill.
These are of course extreme examples, but they do demonstrate how the format impacts the question's answer:
1. The more dominant a single deck is, the more playing the best deck becomes important.
2. The more extreme win percentages there are, the more the metagame becomes important. This means lots of 80/20 or 90/10 matchups.
3. The less of (1) and (2) you have, the less the question matters and the more luck and skill matter.
(1) and (2) are not mutually exclusive. You could have a format with an academy-like deck, but another deck that beats it all the time but loses to everything that isn't academy. In this case both playing the best deck and the metagame will be highly relevant to your success. How much each factor matters, and which one matters more, depends on the particular format's win percentages.
Now let's look at type 1. Our format is fairly balanced (the BBS's and long's and gat's quickly get restricted back into obedience). Hulk and slaver might be the best decks right now, but not by a huge margin. The nigh-unwinnable 90/10 or 80/20 matchups are few and far between, but we do have plenty of 70/30 or 60/40 matchups. Therefore I think the answer is that both are equally important in type 1. I think you have decks that are good enough to do well most of the time, but at the same time you can have metagames which just totally hate you out. Personally I think that's a great thing, it means the format is balanced. More balance = more play skill matters (also more luck matters, but it's a good trade).