Which Elite Four do you think is the strongest?

[Pokemon Trainer]

For me it's Lance (Gold/Silver) since it was really difficult to defeat
Another difficult one is Drake (RSE) since his Salamence and Kingdra are really strong

 

[Shinso Vampire Nagato]

I always found Agatha to be pretty powerful, she was most problematic E4 I've ever fought actually, well during the first try against the E4. With Drake coming in second, the first time I ever fought Drake was in Emerald, and I destroyed him by accident cause I caught Rayquaza before the E4 thinking it was required by the story...