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Какую форму я должен использовать после давай / давайте?

Какая форма is an object here so it should be in accusative.
The word я is not capitalized (unless it starts a sentence of course).

Давайте пойти is impossible. The other two are possible and have slightly different meanings.

Давайте идти is about the process of walking.
E. g. давайте идти медленно ‘let’s walk slowly’.
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