The only thing that could kill Klaus is the White Oak Stake. However, as seen on TVD, on the off chance someone does successfully take down an Original vampire, the rest of their sire line die, too. So killing Klaus would mean the demise of thousands of vampires. Of course, this is only true until Davina breaks Klaus's sire bond in season three of The Originals. His status as a hybrid makes him difficult to execute, too. Being the Original hybrid, Klaus can kill any vampire with a single venomous bite. The Werewolf blood running through his veins also makes him immune to silver-based magic, meaning the Silver White Oak Ash Daggers can't be used on him. Lastly, the mere fact that he's over 1,000 years old and drinks human blood rather than animal blood makes him incredibly strong, since the level of strength in vampires depends on their age and diet.
