I think DokuWiki has the most readable syntax of all wikis. It was designed to be quickly readable and understandable just by reading the source text. That's also probably the reason why the extension of DW files is simply .txt.

The largest headline (h1) uses the most amount of special characters which are =
The smallest (least significant) headline (h5) uses the least amount of these characters.

====== Headline h1 ======

==== Headline h3 ====

== Headline h5 ==

Markdown:

# Headline h1

### Headline h3

##### Headline h5

PhpWiki:

== Headline h2

==== Headline h4

====== Headline h6

The least significant headlines get the most attention!

I don't want to compare every part of DokuWiki to Markdown or other formats, but there are many more examples which clearly show that people who created Markdown, PhpWiki or MediaWiki didn't really mean to make the source well readable to humans (or they are not humans themselves ).

I mean how could anyone come up with the idea that _subscript should be written with one tilde ~subscript~ and strike-through with 2 tilde characters ~~strikethrough~~ ?!?
Or italic text with 2 apostrophes ''italic'', bold with 3 apostrophes '''bold''' and bold & italic with 5 apostrophes '''''bold & italic''''' ?!?

On the other hand DokuWiki has many missing formatting features, compared to Markdown, for example. But that might be a good thing, DW is easier to learn and use, and for the majority of wiki texts it's all you will ever need.

Interesting footnote: probably the most similar syntax to DokuWiki has Xwiki