• 一人ひとり — one person
• 一番いちばん — number one / the best
• 一部いちぶ — one part / a portion
• 一致いっち — agreement / match
• 一回いっかい — one time
• 一行いちぎょう — one line

When talking about “一日” as a duration — like spending one full day somewhere — the reading is いちにち.

But when referring to the first day of the month, the special reading ついたち is used. This unique reading comes from old Japanese and is still common in everyday speech and calendars.

一人ひとりで 行いきます。
I’ll go alone.

一回いっかいだけ 見みました。
I saw it only once.