1-2-4-All
The most used, and for good reason: it fits anywhere and costs about a quarter of an hour. Everyone thinks alone first, shares in pairs, then in fours, then plenary. The silence at the start is what gives the format its value — without that minute, the group discusses whatever the first person happened to say.
The number of phases doesn't grow with the group, so the format keeps working at a hundred people; the version here is set up for a class or team of up to about thirty, because the shared clustering at the end does scale with the number of groups.