My other vote in this informal poll goes to Don Heck. Heck faced the difficult task of taking over the Avengers shortly after Kirby left the book -- and not only was faced with the task of following a giant, but also had a vastly depowered team to deal with.
While I have maligned Heck elsewhere for his declining skills into the mid-'70's and early-'80's, his work on the Avengers from issues in the late teens through the early #40s (including Annual #1) stand apart as some of his beautiful work. Often called back to pencilling duty later in the book's run, he never recaptured the fine form and wonderful facial expressions that marked his initial tenure.
Always under appreciated, he had a style that perfectly conveyed character nuance during the Kooky Quartet era (and beyond, when Hank, Jan, Natasha and Herc appeared); Heck's style gave an intimate feel to the book. In Heck's hands the Avengers wasn't about fancy effects (a la Kirby) or perfect anatomy (like Big John); it was about the personalities.
For the same reason, Heck was equally suited for the early Iron Man stories (in Tales of Suspense) when the focus was on Tony, Happy, and Pepper. For anyone interested in Heck at his peak, take a look at the early volumes of the Avengers and Iron Man Essentials; you'll get a good sense of his line work and storytelling.