They neither value or respect your opinion.
Lets face it, even the best of friends disagree sometimes. The mature thing to do is to "agree to disagree", but unfortunately toxic friends often stay angry about differences of opinion rather than respecting it and moving on.
They emphasize their successes, but minimize yours.
Whether it is getting into the school you wanted or got the job it's usually small compared to their most recent accomplishment. Equal celebration is a key quality in a healthy friendship.
They always have to be the center of attention.
This can come in a number of forms whether it is being the loudest one in a conversation, getting too drunk, or simply making sure they outshine others. It's fun to be in the spotlight sometimes, but constantly stealing attention from others can translate as insecurity.
They never approve of other friends or significant others, anyone who demands your attention away from them.
It's one thing to be looking out for each other from an outside point of view, but if a friend finds something wrong with everyone in your life that isn't them, it is most likely a form of jealousy.
They constantly point out your faults/failures.
I had this one "friend" that would bring up a failed relationship of mine every time I started to become happy again. It was like she was constantly bringing me down while I was up. If someone feels the need to point out negative points in your life while your feeling positive they are most likely trying to push their unhappiness on you and NOBODY deserves that.
They point out favors they have done for you, but rarely acknowledge what you do for them.
Friends do stuff for each other out of love. It doesn't always have to be equal, but you should be thankful for the little things. This does not just refer to money. Time is just as important and if anyone is giving you their time you should be thankful. A friend that constantly acknowledges things they do for you without acknowledging what you do for them is most likely trying to make you feel as if you owe them something.
They always find an excuse to get angry because they thrive on negative energy.
Sometimes people argue over simply things. If this becomes constant where the bad times outweigh the good then it is definitely a problem.