いずれにせよ

Meanings

usually written using kana alone

Used to introduce a conclusion or statement that holds regardless of the preceding circumstances. Often appears at the beginning of a sentence.

どちらにせよ
いずれにせよ is more formal and often used in written or business contexts, while どちらにせよ is slightly more conversational and typically limited to two options.
とにかくN3
いずれにせよ is a more formal, logical way of saying 'in any case' than the everyday とにかく.
ともかくN3
いずれにせよ is a more formal, logical way to say 'either way, be that as it may', whereas ともかく is casual and conversational.
どうせN2
いずれにせよ is a formal, neutral way to say 'in any case', lacking the emotional resignation or scorn that どうせ often carries.
いずれにしてもN1
いずれにせよ is a slightly more formal, written variant of いずれにしても with the same meaning of 'in any case'.

Derived from いずれ (which, whichever) + に + せよ (imperative form of する, 'do'). Literally 'whichever it may be'.