It’s hard to name a single “most powerful” tactic in chess — positions differ. But if you want a tactic that is purely forcing and often leads to decisive attacks, Double Check is right at the top.
A double check occurs when two pieces check the king at the same time. Unlike a normal check, you can’t usually block it — and you can’t capture both checking pieces.
A double check often happens via a discovered check — one piece moves to give check while uncovering a second check behind it.
The quote “chess is 99% tactics” is commonly attributed to Richard Teichmann. Whether the exact wording is perfect or not, the message is practical: most games are decided by tactics being noticed (or missed).
Full discussion: Is Chess 99% Tactics?
A simple “starter set” many coaches use is: fork, pin, skewer, and discovered attack. These appear constantly and combine into many stronger ideas (including discovered checks and double checks).
Online this phrase is used in more than one way. In tactical calculation, the common idea is: scan forcing moves first — Checks, Captures, Threats — often written as CCT.