A useful way to determine the strength of any relationship is to realize how unbearably irritating you find the participants hen they're together. JD and Turk and without a doubt best buds. They're also obnoxious, loud, inconsiderate, and dumb. It's really fucking inspiring.
Cory and Shawn don't have much in common, but have forged a deep understanding over time despite that. They say opposites attract, which would greatly apply to them if not for the fact they are two undisputedly heterosexual males who are definitely not attracted to each other in any way, sexual or otherwise.
Proof that sometimes being a good friend means going the extra mile (or, like, hundred thousand miles. It's hard to tell. The maps are weird), Samwise and Frodo have faced it all together: certain death, the murder of their guide and mentor by a gigantic fire demon, and repeated attacks by some weird grey mountain monster who they believe unreservedly to be an ally. As meaningful as it was for them to go it alone, that last one would definitely not have gone down if they'd stuck with the gang.
It's pretty obvious to the casual observer that being friends with these guys is more trouble than it's worth. Not in many other high schools does getting in with the cool kids mean you run the risk of giant snake murders/horrifying despair ghosts/literally the most evil creature in existence. It's really impressive they didn't ever fall apart or just die or something. probably helps that two of them secretly wanted to have sex all along, too.
We just touched on "secretly wanting to have sex" as a motivator for friendship. Never has that been truer than with the Friends, the majority of whom spent all their time either having sex with each other or making jokes about having sex with each other. But that's the kind of thing you can expect when you drink as much coffee as they did. While you could pick certain standout pairings from the group to focus on, what really made the Friends special was their entire dynamic. They're not the best best friends because they care more or listen better. They're the best best friends because they all equally contribute to one big ol' entity of love. Except Monica. Monica's kind of a pill sometimes.