some confusing expressions with number

When I want to express one of the series in harry potter, which one is better? ex) Third episode is my favorite among whole series.

Third episode/ episode three

Are all of them can be used? If not, what is the problem with them?

One more question... In a subway linemap, what is the exact way to express line?

line one = the most common used first line (is it also possible?/ then how about line first?)

I want someone to distinguish their differences as I don't have intuition.

• Does this answer your question? A question about using definite article with ordinal numbers: "content with third place" or "content with the third place"?. Basically, I watched episode three is fine, but you need to include the definite article if you use the "ordinal" form I watched the third episode. May 12, 2020 at 12:12
• I mean the order of those words. the third episode is the same as episode three? Do they mean the same?
– Yemo
May 12, 2020 at 13:37
• Yes, they mean the same thing. But to repeat myself, when you say I saw episode 3 there's no article, but you must include the definite article when you say I saw the third episode. May 13, 2020 at 14:40