In Vedas, Vritra is the strongest demon and its not even close .He was a threat which was devouring the cosmic order RTA .
In ithihasas its easily Madhu and kaitabha . No asuras comes close to them in power. They operate on much higher cosmic scale
Earth was created from using their fat .
- In those ancient times, I (Brahma) was born from you. You used your maya to generate the handsome serpent Ananta who lies down in the water and two extremely strong creatures. They were Madhu and Kaitabha and their bones covered everything and constituted mountains. The earth was formed from their fat. I was also born from a divine lotus, as resplendent as the sun, originating in your navel. You said that creation was my task and invested everything in me. O lord of the universe! Having taken up the burden invested on me, I worshipped you.
Valmiki Ramayana (7–94)
2: Hiranyakashipu was the strongest with boons. Since he literally became the universe as per Mahabharata/harivamsa
- भवेयमहमेवार्कः सोमो वायुर् हुताशनः ।
- सलिलं चान्तरिक्षं च नक्षत्राणि दिशो दश ॥
- IAST:
- bhaveyam aham evārkaḥ somo vāyur hutāśanaḥ |
- salilaṃ cāntarikṣaṃ ca nakṣatrāṇi diśo daśa ||
- Literal:
- “May I alone become the Sun, the Moon, Wind and Fire, Water and the atmosphere, the stars and the ten directions.”
All the other fodder rakshasas haven't showed any power anywhere near cosmic scale even with boons which makes all of their accomplishment such as defeating gods useless in comparison to shere power possessed by hiranyakashipu. Any random devata like Vayu would instantly kill them easily if they are allowed to
- ततः स दृष्ट्वा निहतं हनूमन्तं महाबलम् ।
- वायुः प्रचण्डः प्रज्वाल्य जगाम त्रिदिवं तदा ॥
- Vāyu became extremely angry and ceased to blow in the three worlds, causing the gods and living beings to suffer.