Well, it really depends on the format as some of you have pointed out.
For me, these are the most lethal attacks:
Unlimited:
Sneasel's Beat Up just like Kyogre said. Turn 2, you can do a max of 140 damage and your damage is around 80 on the average. This can happen especially if your opponent is dumb enough to accept the Challenge! card which I use with Wiggly + Sneasel.
If it's not this, I'd say it should be Feraligatr's Riptide (which I barely survived in one tournament) or something that does can do 70-100 damage every turn, like Riptide.
Modified:
Volcanic Ash may be deadly. Well, it is, considered that it's guaranteed. Pokemon like Linoone and Rayquaza can do a whopping amount of damage, but only if you're really lucky in flipping coins. So I guess Volcanic Ash is the best OHKO'er (not considering Burn Down because of the energy required), Pollen Hazard is fun to use, and Triple Poison is just plain EVIL.
Regarding Destiny Bond, it's a good stall attack but it won't get you anywhere. If your opponent decides to sacrifice his or her Pokemon anyway, Destiny Bond is useless. Also, about Strike and Run, it really is a good attack, but I don't think it is in any way lethal. The threat to your opponent there is the Pokemon you put on your bench, not the Strike and Run itself. It's great, but not lethal. Just my opinion...
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