The Kikoho (Tri Beam in the American version). Some call it the "Greatest DB Technique Ever." Well, not actual, but I think it is (yeah, I know, I think everything about Tenshinhan is the best). But seriously, the Kikoho is a massive offensive ki energy attack in DB, that has helped Ten and the gang out a few times. This is probably the most powerful attack in DB (aside from the Genki Dama, where Gokou collects others' power). But since the Kikoho uses all of your ki it is definitely the strongest attack in Dragon Ball.
Tenshinhan popularized this wonderous attack which was taught to him by Tsuru. It has come in handy for him quite a few times. It probably won Tenshinhan the 22nd Tenkaichi Budoukai, prolonged Cell's final transformation (who knows what would have happened if Cell was in his Perfect form before he actually was!), It saved Dende and Mr. Satan's life (What would happen w/out god?), and has helped out a few other times. The Kikoho is a last resort attack, because it will probably kill you if you use it.
Kikoho and Shin-Kikoho
It's Tenshinhan's signature technique which he invented. He concentrates all of his ki and energy into his hands and releases it in a huge energy blast. The attack can kill you if you use it too long, because of the immense pressure it puts on your body. Tenshinhan puts his fingers and thunbs together about chest-high and forms a kind of "O"ish triangle and then delivers the enormously destructive blast. When you hear the term "Shin" Kikoho, it is the newer modified, more efficient, more powerful, version of the Kikoho. Only Tenshinhan knows the attack, which gives Tenshinhan something to be proud of. If Gokou knew how to use the Kikoho, he could kick some even more butt, because of his high PL.