5. Crystal (Crystalia Amaquelin)
Crystal is an Inhuman who can control the four elements and was also a long-term substitute member of the Fantastic Four before she joined the Avengers. Although she only found her way into the team simply by holding hands with Quicksilver, she soon was inducted into the Avengers roster on her own merit when Pietro did one of his trademark abrupt exits from the team (I believe to go off and play with X-Factor?). Crystal stayed by the team for a surprisingly long tenure from 1992-1996. Despite four years of activity alongside the team and being present for such events as Operation Galactic Storm, the Infinity Gauntlet, Citizen Kang, Bloodties, The Crossing and Onslaught, her contribution to the team has been all but forgotten.
Crystal currently appears alongside the Inhumans and recently interacted with the Avengers during Realm of Kings.
Sersi is an Eternal with a few tricks up her sleeve. Sersi kinda loitered around the team for a while after Inferno and was added to the active roster when the team line-up was reshuffled when the Avengers U.N. Charter was implemented. She stayed with the team until 1994 and most notably was the target of the Gatherers, a dimensional-hopping team who sought to kill every Sersi in existence. Sersi had a long tenure as a prominent figure within the team but her relevance and importance has been downplayed since.
Sersi regularly appears alongside the Eternals whenever they show up, most recently in Hulk #49.
3. Quasar (Wendell Vaughn)
Former Marvel Boy, Quasar wound up on the team under similar conditions to Sersi after having hung around them after the Inferno cut down their membership. Quasar was the "every-man" superhero and was just a really nice guy. It was hard not to like Quasar as a character but that was his downfall. Despite being utterly likeable, his contributions to the Avengers aren't entirely memorable and he tended to change costume quite frequently which didn't help. Although he played a prominent part in the team between 1990-1992 and was a key character in Operation Galactic Storm and Infinity Gauntlet, in the present day, as an Avenger he seems to have been relegated to the obscure corner alongside Crystal and Sersi.
2. US Agent (John Walker)
When people think of "the Top 30 most Essential Avengers", they probably wouldn't think of US Agent which in my opinion is a big mistake. Unlike his friend and colleague War Machine (who hasn't stayed with the Avengers for more than five minutes really), US Agent has served with countless incarnations of the Avengers from the West Coast team, to the main Avengers and the "Avengers-in-spirit" Force Works - even leading an Avenger team on two seperate ocassions. Walker was initially forced onto the West Coast team (which led to Hawkeye quitting) and began leading the team, albeit not very well. He was soon welcomed onto the team properly and stayed with the core team until they gave up when Force Works dissolved. He rejoined with the Avengers on-and-off, mostly notably during Maximum Security and Dark Reign and has totaled a surprising 11 years as an Avenger! Despite his commitment to the team, he's not given the respect he deserves as an Avenger perhaps because he always be living in the shadow of Captain America.
1. Captain Marvel (Monica Rambeau)
On the surface it's very easy to hate Monica Rambeau. Her history reads like a classic case of a writer having a pet character and forcing her into major prominence in a book they're writing. Roger Stern created the new Captain Marvel (who actually had nothing to do with the original) in a Spider-Man Annual and she soon found herself on the Avengers which Stern was also writing. By the end of his run, Captain Marvel had risen the ranks and was leading the Avengers! Despite the quite obvious nepotism towards his own character, fans took to Captain Marvel because she was an actually likeable and strong character and Stern was a very capable writer. When Stern left the book, Captain Marvel vanished although made a brief return a couple of years later as a Reserve Avenger. She was an Avenger from 1983-1988 and a prominent reserve during the "Charter Days" most notably taking part in Galactic Storm. Captain Marvel is a long-serving member, as well as a former leader, and yet despite this appears now as nothing more than a footnote on Avengers history.
Monica Rambeau changed her superhero name to Photon and then later Pulsar and after joining NEXTWAVE effectively vanished.