To me, most dragons don't look that tough; Garchomp is pretty thin but just covered with spikes and fins, so I'd assume it was mostly fast, Hydreigon looks round and hairy, Salamence is fat, same with Dragonite etc.
Its likely that only because we know Dragons usually have high base stats that we're picking them, but for me pic related always stood out as looking strong as fuark.
It looks bulky and solid, so I'd assume it'd have tanky defences. Its got huge arms, all those vein-straps and its carrying two huge concrete pillars as a show of strength, so I (rightly) assume its also unbelievably strong.
It also looks slow as hell but with the assumed bulk and strength, that wouldn't be problem.
I wouldn't say it looks the "strongest" in the world of pokemon, but it looks like the concept of strength as a pokemon. Everything about its suggests "big and powerful".
>inb4 dick or mom jokes
