If you want to congratulate a colleague on a promotion, the best approach is to:

Directly emailing the person you wish to congratulate is the most thoughtful and personal approach. By sending a direct email, you create a space for a more intimate and specific message that acknowledges the individual's accomplishment. This method allows you to convey your congratulations in a way that is encouraging and meaningful, giving the recipient an opportunity to appreciate your acknowledgment personally.

Using Reply All to a company-wide email can result in your message getting lost among many others, diluting the personal touch and possibly overwhelming the recipient with additional messages. Conversely, replying to the original email does not provide the space for a personal message and may also disturb the thread with multiple responses. Congratulating someone in a meeting, while public and celebratory, might not offer the same level of individual recognition or encourage a meaningful exchange, especially if time is limited in that context. Direct communication is generally seen as more sincere and effective for personal milestones such as a promotion.