Concerning Blake's, "The Tyger," one doesn't normally talk about which literary devices are the "strongest." I can point out literary devices that are used and are central to the poem, but I can't guarantee that they are the "strongest."
The theme or idea that the same being that made the lamb made the tiger is central to the poem. Blake is dealing with perceptions and the elimination of dichotomies. The lamb and tiger are not created by two different beings, but by the same being. Thus, the being that made them has something of the lamb in him, but also something of the tiger. By extension, then, all of creation, humans included, have the lamb and tiger in them, as well: they are just two sides of every being.
Imagery is also central to the work. You can pick out virtually any line in the poem and find powerful imagery. The image of God hammering out the frame of the tiger on an anvil is a powerful one, located in stanza four.
Finally, allusion plays an important role in the poem. Stanza two includes an allusion to Icarus, who in classic myth attempts to fly to the sun ("...what wings..."). The allusion mixes with the fire imagery, and the idea that the creator forges the tiger in fire, like a blacksmith.
Stanza five contains an allusion to the fall of Satan from heaven (the stars throwing down their spears), and the speaker asks if after the fall of Satan, did the creator look on the tiger and smile.