In the Marvel Universe, the big three superhero teams are the Fantastic Four, the Avengers (including various incarnations like West Coast Avengers) and the X-Men (with related teams X-Force, Excalibur, etc.). There were other teams also, and of those, my favorite has always been the Defenders, the "non-team" that had its heyday in the 1970s and `80s and is probably the biggest second-tier team (the Champions and Infinity Watch are definitely less memorable).
Essential Defenders Volume 2 begins to really give the team its identity. In previous issues, it featured several heavy hitters (the Silver Surfer, Sub-Mariner, Hulk and Dr. Strange), all of whom were not team players. Eventually, as this volume kicks in, the Silver Surfer and Sub-Mariner have gone their separate ways and have been replaced by Valkyrie and Nighthawk. It's these new heroes who help give the team more character. Also, this volume features the beginning of Steve Gerber's writing run; Gerber, also the creative force behind Howard the Duck, would give the Defenders a little bit of an off-beat quality (although, admittedly, this happens more in later issues).
In this set, the Defenders take on several big villains like Magneto, the Enchantress and the Badoon, along with lesser know villains like the Sons of the Serpent and Tapping Tommy. There are plenty of guest heroes too, including Son of Satan, Luke Cage, Spiderman, the Human Torch, the Thing, Daredevil and the Guardians of the Galaxy.
While it might be too much to say that this is really great writing and art, most of the material is at least good and it's all fun reading. For another look at what made Marvel entertaining thirty years ago, this is a good book to pick up.