The Brahmashira astra is deemed invincible and unstoppable even for Krishna.
When Ashwathama' was about to fire it against Pandavs, there was nothing stopping from Krishna's interference. The war was over and it was simply a massacre of Ashwathama'. Krishna could've intervened if he could, but instead told arjun to use the same weapon against it to save their own lives.
‘O Arjuna, O son of Pandu, the time is come for the use of that celestial weapon which is in thy memory, knowledge of which was imparted to thee by Drona. For protecting thyself as also thy brothers, O Bharata, shoot in this battle that weapon which is capable of neutralising all weapons.'
However Vyas and Narad intervened citing if the weapons clashed, it would cause tremendous collateral damage possibly consuming both of them. Hence Arjuna withdraws it, but A'thama was incapable of it and therefore fired it against Abhimanyu's child in Uttaras womb.
This again wasn't part of war, neither was the weapon under any boon and so nothing was preventing Krishn from stopping it.
Instead Krishn says it's invincible and cannot be stopped by any means, it will accomplish the objective(of killing Uttaras child).
"The fall of this mighty weapon will not be fruitless. The foetus will die. But being dead, it will live again and have a long life! As regards thyself, all wise men know thee for a coward and a sinful wretch!
Featwise, this weapon stands at the top. However as per hollow statements, Vaishnavastra and Pashupatastra are on similar tier. Vaishnavastr said to be capable of slaying Indr and Rudr and Pashupatastra capable of slaying Brahma and Vishnu.
For invincible warriors, there are none. All were defeated at some point or other.