The Defenders were formed in 1969 by Doctor Strange in issue #183 of his own comic, Sub-Mariner #22, and The Incredible Hulk #126.  Strange’s own series had been canceled, and Marvel tried to continue the life of the character in a team book.  The next appearance of the team was in 1971, in Marvel Feature #1, and the book sold well enough that the team got their own book.  The Defenders never got the respect of teams like The Avengers or Justice League, but they didn’t have to–the
title was more quirky than important, more mystical
than super.
But perhaps the most important thing about The Defenders was that it was the best place to find a “mainstream” version of Steve Gerber’s incredible imagination.  I recognize that his more esoteric works like Howard the Duck and Omega the Unknown aren’t for everyone, but in The Defenders he was able to merge bizarre, random concepts like Elf With A Gun with team-ups with The Avengers.
Click the links below to see panels from (just about)
every issue of Volume One of The Defenders.  (The other volumes really aren’t worth reading.)

Roy Thomas Creates the Team.  Grade: B.

Team’s first appearance in Doctor Strange, Sub-Mariner, and Hulk

Sub-Mariner #34-35 (second time Hulk, Strange and Subby got together)

Marvel Feature #1 (first appearance under the name “Defenders”), 2-3

Steve Gerber takes over the writing chores.  Grade: A+

12=Len Wein

Giant-Size Defenders #2

17, 18-19 (featuring Power Man!), 20, 21, 22, 2324,

Giant-Size Defenders #3#4

25 (first appearance of Elf with a Gun)
26 (with Guardians of the Galaxy)27 (first appearance of Starhawk)2829-30, 31, 32-33 (first appearance of The Headmen)3435
Howard the Duck Treasury Edition #12 (the team teams up with Howard)

36, 37, 38, 3940, 41

(Mostly ) David Kraft (Kraft picked up the book without cowriters at #49), Keith Giffen (pencils), and Klaus Janson (his first work as an inker).  Grade: B

42-43, 44-45, 4647-4950, 51, 52, 53-56

Fill-in by Gerry Conway, Chris Claremont, George Tuska and Dave Cockrum.  Grade: C+:  57
David Kraft and Ed Hannigan.  Grade: B+

58, 59-60, 61, 62-65 (first “old order changeth” issue), 66-68

Fill in by Mary Jo Duffy.  Grade: F
.  69
Ed Hannigan (writing) and Herb Trimpe.  Grade: D.

70-73, 74-75, 76-78, 84-8688-91

JM DeMatteis and Don Perlin.  Grade: A-

92, 93, 94 (first appearance of Gargoyle), 95, 96 (vs. Ghost Rider), 97-99 (The Six-Fingered Hand storyline), 100, 102, 103, 104, 107-109 (Valkyrie dies), 105 and 111 (two issues about Satan), 112-114(Squadron Supreme), 115 (the Dr. Seuss issue), 116-124 (the end of The Defenders), 125126 (The New Defenders)
Issues not worth reading: 
127-131, 132-135, 136-183, The End (where nearly everyone dies)

The Defenders’ Base, from #50