The strongest spell I've ever done? I would consider every spell I've ever done the strongest. All of them worked to different degrees. All of them had different consequences as a result of me working them.
The matter of spells for me, or even regular workings, isn't something simple, as much as it is simplistic. I build the working I want in the way I want, I perform it, I take responsibility for what will come next, and I move along, workng with it physically as much as possible. If I can't work with it physically, I will simply leave it alone and let it occur in its own time.