What should you promote a pawn to?

King or Queen, whatever is available

Imagine someone believing that haha.

Hi.

That depends on the particular position.

Most of the time, it is best to promote to queen.

Sometimes, it is better to underpromote. Knight and Rook are next in order of frequency. Bishop underpromotions are extremely unusual.

And sometimes, it is better not to promote at all (or leave the promotion for later).

I remember one game where the best move of my opponent was to underpromote to a knight (it was his only drawing move). He did not consider it, did not promote, and lost. Oh, in that position, a queen promotion would have been met by an immediate knight fork (my knight attacking his newly promoted queen and his king).

In another game, I underpromoted to a rook, to avoid a stalemate.

I do not remember any other underpromotions (or potential underpromotions which were also good moves) in my games.

Greetings.

Queen, most of the time. Maybe promote to something else to check the king in case there is an enemy rook on the back rank.