Inspired, to no one’s surprise, by this thread.
In my first post in the other thread I mentioned Tradd from The Lords of Discipline. I start this spin-off by mentioning another character from that novel: Mark Santoro. He stands by Will long before he has too and for long after most people would have; he is a far better friend to his roommates than any of them deserve. The fact that he dies in 'Nam is the most unjust thing that happens in the story.
Anybody else?
Can we add Watson and Holmes here? or are they to dominate the frenemies thread?
Nero and Archie. Sure, they snip at each other constantly, and Archie alternates between getting fired and quitting every other week, but deep down, those two share an astonishingly strong mutual trust, respect, and friendship. Its always a treat the few times those two let how they feel for each other peak through.
Spock.
Jack Aubrey & Stephen Maturin.
3 and 5 beat anything I’ve got.
There is always Hornblower and Bush. There is the escape from France with Bush injured, and when Admiral Hornblower is tasked with recovering a mutinous crew and ship, the first thing he says is, “I’ll have Bush.”
Frodo and Sam.
They’ll go in the sure-to-be-upcoming Best “Just Friends” in Literature thread. If you know what I mean. Not that there’s anything wrong with that.
Jack Sawyer and Wolf (The Talisman, by King and Straub). That’s a friendship that gets me every time.
oops, wrong thread
Friend to whom?
I guess he meant that for the worst best friends thread.
Anne Shirley and Dianna Barry. YA, yes, but they will always define the perfect friendship to me.
And, along those lines, Trixie Belden and Honey Wheeler.
The Ya-Ya Sisterhood. If that’s literature. . . but yeah, they are the sort of friends I’d most like to have, and I can’t think of any others who have hung together through more serious crapola.
I’d put them on this list. Holmes and Watson ≠ House & Wilson.
Sydney Carton? Not many people would give up their head for somebody.
How about Harry Potter and Hermione Granger?
Nita and Kit from Diane Duane ‘young wizards.’ Yeah, they’ve had a few bad patches, (mostly where the plot required it,) but it’s obvious how much they’d sacrifice for each other if they had to, and I’d love to have that sort of an unspoken bond with a good friend.
Honorable mention also to everybody’s ‘little buddy’, Piemur of Pern. 
Gilgamesh & Enkidu
Achilles & Patroclus
After Achilles death, the Achaeans burned his body and mingled his ashes with those of his friend Patroclus; both were buried together on the White Island.
The similarities between Gilgamesh/Enkidu and Achill/Patroclus are probably no coincidence; many friendships between heroes in literature were influenced by, sometimes copied from the earliest model.