What do you think is the best format for configuration files?

YAML would be almost perfect if it allowed tabs and their reasoning against them is stupid IMO. The main argument against YAML is that it is difficult to parse, but there are plenty of high-quality parsers.

TOML just looks ugly and confusing when it comes to more complex things but looks great for just strings.

JSON is worst IMO because of lack of comments. I think YAML is a lot more readable than JSON but some people like the strict requirements.