What is the hardest grade bolt you can buy?

I'm curious to know, what would be considered the toughest bolt available on the market today in terms of grade and durability? Are there specific standards or ratings that manufacturers use to designate the strength and quality of bolts? Is there a particular grade that stands out as the most resilient and reliable for heavy-duty applications? I'm looking for a bolt that can withstand extreme forces and conditions, so any insight you can provide would be greatly appreciated.