1. Howley
Each year in the regular Hall of Fame voting, a player has to make the preliminary list, then the semi-final list, then the final list, then be one of the seven players who get voted in. Howley was probably a better LB than any of these Hall of Famers, and he never made it past the preliminary list in the voting.
Robinson: 1 AP All-Pro, 2 rings
Jackson: 0 AP All-Pro, 0 rings
Tippett: 2 AP All-Pro, 0 rings
Wilcox: 2 AP All-Pro, 0 rings
Howley: 5 AP All-Pro, 1 ring + SBMVP in a loss, all-time leader in SB takeaways
Everyone (coaches, players, media) considered Howley the best at his position during the time he kept making those All-Pro teams. He is the only eligible player who made at least 5 Pro Bowls, was a Super Bowl MVP, and is NOT in the Hall of Fame. You could make a case that he's the most underrated NFL player ever.
2. Martin
A borderline HOF candidate, Martin was a 3-time Pro Bowler, DPOY in 1977 (when he had 23 sacks before it became an official statistic), and one of only five players left on the HOF Selection Committee's All-Decade Team of the 70's who has never been a finalist for the HOF. He's only even made the preliminary list once (2001), which means he went from being considered one of the 45 best players of his era to not even one of the top 150. That's not just "underrated," it's practically "erased from history."
3. Harris
38 of the top 39 vote-getters for the HOF's All-Decade Teams of the 70s and 80s are in the Hall of Fame. The only exception is Harris, who was one of the top 10 vote-getters for the 70s team. Unlike the first two players I listed, Harris was at least a HOF finalist once. Harris' rejection by the HOF committee in 2004 prompted Paul Zimmerman (no afiliation with Cowboys or Harris) to resign from the committee in protest.
4. Woodson
Should have made the All-90's team ahead of Carnell Lake, and was a semi-finalist in the HOF voting this year. John Lynch, another big hitter, made it past him to the final list, but Woodson was a big hitter who could cover.
5. Pearson
The only one of the 22 position players on the All-70's first team who's never been a HOF finalist, and he's never even been a semi-finalist.