Artis Gilmore
There’s no doubt that Gilmore is an NBA legend but he never won a Finals. However, he did earn a ring when he played for the Kentucky Colonels in the defunct ABA. Then he joined the Bulls and became a notable figure in the league. He made six NBA All-Star appearances and five during his time in the ABA.
The A-Train had a magnificent on-field scoring percentage during his prime and became a fan favorite. The only time he made the NBA Conference Finals was at the tail end of his career with the Boston Celtics. We assume that the privilege of winning a ring with the ABA eases the pain of missing out later (via USA Today Sports).