"Good news, everyone!" by far.

A common way to get around explicitly giving the HP of a monster and telling them nothing is the "They look... " rule. When they ask how many HP the baddie has left, tell them "They look injured, but not enough to hinder them" or "they look bloody and totally messed up" etc. As a rule of thumb, you can decide their health into quarters and come up with a common phrase for each, or come up with them on the fly depending on the situation: "Grog's hammer has left some of its ribs broken, but it looks healthy enough to keep fighting for a while."