What is the most important piece of information in a X.509 certificate? X.509 has a structure which contains information like version number, serial number, signature algorithm, issuer, subject name, subject public key, signature of the issuer etc.  

However, I'm having an argument between friends because according to them, the public key is the heart of the certificate. 

On the other hand, I think issuer's signature (CA) is the most important attribute. This is because basically you can't do anything without having a valid certificate, whereas if public key for some reason is missing, I still think you can digitally sign documents (since it uses private key and not the public one). 

Certificate here comes handy because you need the subject's name (signed by youremail@gmail.com") which you can access since you have a valid cert. If you don't have the CA's signature, you can't even digitally sign.  

I need to know which is the right answer. Thanks!
As fgrieu says in his comment, the answer to this question is ambiguous; it depends on what you use the certificate for.

You can perfectly trust a certificate, without having a CA signature. A CA signature only signifies that whoever trusts the CA, will also trust your certificate.

The public key however is the way with which the owner of the certificate and key will identify their self with. Therefore, I would say that the public key is more important, because a certificate is useless without it.


  This is because basically you can't do anything without having a valid certificate, whereas if public key for some reason is missing, I still think you can digitally sign documents (since it uses private key and not the public one). 


Without the public key, nobody will be able to verify your signature, so your signature might as well be random noise.


  I need to know which is the right answer. Thanks!


As I said, there is no "right" answer here; apart from that you actually need a public key to be in there.