Last year, I was affected by an intense bout of depression, that eventually resulted in me being rushed to the hospital from an attempted suicide. The result of this was that one of my friends cut contact with me completely. While I was in the hospital, unable to defend my reputation, he then proceeded to spread lies about me, in order to convince others to cut contact with me as well. In the end I was left with only one friend at a time when support could mean the difference between life and death. Luckily I am still here.

This remaining friend of mine is still in contact with this person, and has even made him the best man at his wedding.

If he on the one hand admits that his friend's actions were wrong, but on the other hand, refuses to express his opinion on such matters, even going so far as to make this person his best man at his wedding, should I be offended? Should I consider it an insult to my value as a human being? Should I end my relationship with this friend, because his actions are harmful to my sense of value?

I'd recommend checking whether what you perceive as "spread lies about me, in order to convince others to cut contact with me as well" may have been a fairly accurate assesment. You probably have much to learn and this is a big opportunity, don't throw away your only friend.

Please don't cut ties with your soon to be married friend, he might have to get burned himself before his eyes are opened to the other friend's nasty streak.
Harbouring feelings of bitterness and revenge will bring you isolation and unhappiness. Be the bigger person. Take the opportunity to be noble and move on with your life.
This man's attempts to poison your reputation were probably driven be jealousy or insecurity, pitiable isn't it? You're fortunate at least to have seen his true colours and can avoid ever having close ties with him again.

You should recognise that the loyalty and commitment which your one remaining friend showed to you also account for why he retains his friendship with the guy who let you down. He is a man who does not abandon his friendships lightly. You, of all people, should be slow to punish him for that.

You get to choose your own friends, but you do not get to choose your friends' friends. You cannot make his friendship with you conditional on his abandoning other friendships.