This is a question without an answer, because there is no such thing as a "strongest" shout. Every shout has drastically different effects suitable for different situations. It's hard to even say whether there is such a thing as a "most useful" shout, since depending on your playstyle you may find more use out of one over another.
Soul Tear is a personal favorite of mine - it dishes out 300 damage, and if the target dies, it also soul traps them and reanimates their body to serve you for 60 seconds. Not sure if you can call it the "strongest" shout, but it's definitely one of the coolest.