Which one should I use for page title?
- Apple — Tasty Fruit
- Apple — A Tasty Fruit
- Apple — The Tasty Fruit
The article is only about tasty apple.
[Updated after checking with this page. (Thanks, J.R. Have you not shown me the link, I wouldn't have known about it)]
If you want to emphasize that apple is delicious and in a way unique, it would be more appropriate to use the definite article, the.
So I would go with option 4.
However, if you are not planning to emphasize too much about apple being a tasty fruit, you should use the indefinite article, a.
It would make apple sound less special in terms of being tasty.
When it comes to capitalizing words in titles, I would follow the capitalization rules and not capitalize prepositions, articles and conjunctions unless they are at the start or the end of the title, or they consist of more than four letters. In this case, the noun phrase after the dash plays the role of a subtitle, as suggested by J.R. and Lie Ryan, and therefore the first word has to be capitalized, regardless of whether it is an article.
Note: Some sources may agree on the capitalization of prepositions. But I personally don't.
You can consider replacing the dash with a comma.
Subtitles are usually separated by colons, and subtitles that start with "a", "an", or "the" is usually capitalized (as exemplified on here). Thus, we are left with three choices:
Which one you choose depends on what you want to emphasize and on your own "taste" (both linguistic and gustatory). Using the article "a" implies that apple is a tasty fruit, but not necessarily "the" only tasty fruit nor "the" most tasty fruit. That's the reason why I would avoid "Apple: The Tasty Fruit", though you might want to use the article "the" if you want to put an extra emphasis on the tastiness of apples.
The zero articles (i.e., using no articles) are usually used for plurals and mass nouns in normal text. I see no reasons why the rule would be different for titles. Therefore, if using zero articles, it should be "Apple: Tasty Fruits" or "Apples: Tasty Fruits".
In this particular case, I'd choose between either "Apple: A Tasty Fruit" or "Apple: Tasty Fruits".
Of your options, 1, 2 and 4 work as they have consistent capitalisation, however you might be better off using
Apple - a tasty fruit
Apple - the tasty fruit