Henry Aaron is about the least surprising player possible to start an alphabetical list of HOFers. But before I talk about his Replay League career, and since we just lost him recently, I encourage you to take a look at his Baseball Reference page linked above. I look through the page of every rookie player when I prepare for league drafts, and Aaron's page was the only one that made me stop and stare in awe. The guy was an MVP candidate for 19 consecutive years! He's not the best player in baseball history (Babe Ruth) or even the best player of his era (Willie Mays), and he isn't the most important player in history (Jackie Robinson, or maybe Ruth). But he was historically great at everything, on and off the field. I would claim that nobody in baseball history was both as great as he was on the field, and as important as he was off the field. I am too young to have seen him play, but I think he is my favorite player of all time anyway.
Back to RL...Hank Aaron was actually not that great of a hitter. I mean, he was very good, but he only hit .276 over his career, with 363 HR. He won one batting title and led the league in HR once, with a career-high 33 in 1960. His career OPS+ was 119, so good but not HOF-worthy for a corner OF. He made only 5 all-star games, the last in 1961.
So how is Aaron in the HOF? Defense! He won 12 Gold Gloves in RF, and was 282 runs above average over his career - almost 30 wins just from defense! In his best years he was an MVP candidate, though he never won or deserved to. But most of the time he was a good hitter with great defense, and he played for quite a while. He's not inner circle HOFer, but he's in nonetheless.
Funny story ... he was kind of crummy in 1966, and the Cardinals (his only team to that point) released him. The Angels picked him up, where he had a solid year at age 33. After the season, the Angels traded him back to St. Louis for Ken McBride, an oldish SP. Aaron played 2.5 more solid seasons for the Cardinals before finally getting traded to the White Sox to finish out his career.