Here is an excellent poem that builds up to a climactic answer.
I once explained it to a group of kids in Hyderabad, leading them through a bunch of questions to the answer. They loved it.
पृथ्वी शेषभृता, स शम्भुमकुटः, कैलासवासोप्यसौ
कैलासोऽपि दशाननेन तुलितः, बद्धोऽप्यसौ वालिना ।
वाली राघवविष्णुना युधिजितो, विष्णुः सतां मानसे
तस्माद् विष्णुपरायणाद् गुरुतरो नान्योऽस्ति लोकत्रये ॥
The question it pursues is, 'who is the heaviest/strongest in the world?'.
'The Earth', you say? But Adi Sesha is carrying the Earth on his hood.
Shambhu (Siva) wears him on his crown as an ornament to his locks. That makes it Shiva? Not yet.
Shiva is carried by Kailasa parvatam, which in turn was raised by Raavana.
Vaali carried Raavana in his armpit once upon a time and circled the entire globe.
Rama killed such Vaali with just a single arrow.
Such a valiant Rama, who is Vishnu himself, gets caged in the heart of his devoted bhakta.
Hence the conclusion is, Vishnu bhakta is the strongest/heaviest of all, since he's carrying all the others mentioned so far in his heart. :-)
What is bhakti, but love?
Hence, love is the strongest force in the Universe as it keeps the most powerful in its control.
