noun
number one; item 1
Used as a label for the first in a numbered series, such as a building, room, document, or list item.
会議は一号室で行います。
The meeting will be held in Room 1.
資料の一号を確認してください。
Please check item 1 of the document.
一番 means 'first' or 'best' in a ranking, while 一号 is a label for the first item in a numbered series.
Compound of 一 (いち, 'one') and 号 (ごう, 'number').