Difficult question and I honestly don't want to narrow it down to 1 best quality. There are different types of friends, too. Instead I'll describe what makes a great friend from my perspective. Wall of text incomming.
- Someone who sees, accepts and likes you for what you are, yet pushes you forward and supports you, motivates you and shows you new things, new perspectives, reflects you.
- Somoene who you can fight with and know you'll be able to make up and talk about it. The trust, that you can speak your mind, the knowledge that you can mess up and show your bad sides and it'll still be fine, allowing you to truly relax.
- Someone, with who you can simply be yourself and fool around.
- Someone who sticks with you during your bad times and cheers you up.
- Someone, with who you can grow together as a person.
- Someone who is interested in your wellbeing, thinks about you and acts in your interest.
- Someone who appreciates you.
None of these come for free though, you draw people towards you with your behaviour, with your expressed views and interests. You'll have to be yourself, not a fake, not an image, and the "yourself" you present has got to be a pretty good friend, or rather person, yourself to build a friendship like that. You get what you deserve and work for (most of the time at least, but in regards to friends, I know there really are some amazing people out there, that'll be your friend and care about you, no matter how much you feel like you don't deserve their presence and how much you feel like a nuisance at times, but tbh, everyone probably feels like that sometimes during their bad times). Small Edit: I just want to add that this perception is likely highly influenced by the highlight reel phenomenon, where you mostly see and remember your own mistakes and other peoples successes, which warps and twists your perception of yourself and others. Many mistakes you and others do go unnoticed, but you do notice your own mistakes.
If any of this sounds like I speak from personal experience, I do. I have some reaaally really good friends and I need to tell them more often how much I appreciate them and you should, too, this goes for any person you care about and hold dear. In regards to the nuisance part, when you have really good friends, they'll remember things and know who you are and don't forget that, even during times when you have it rough. You're not always your best self, some days, weeks or months you can be like a totally different person and constantly get mad, but good friends will still know who you are and keep cheering you up, even if you get mad at them. It's really not easy to find people like this though, so value them highly.
Well, this got out of hand, but that's just how much I love my friends. You really can't take it for granted.
SQ cause my brain too filled with nostalgia to think up a question.