The CW's Arrowverse - comprised of Arrow, The Flash, Legends of Tomorrow and Supergirl (plus, Constantine and Vixen) - is arguably the most impressive TV superhero universe to date.
Over the years, it has grown into a huge expansive world which has brought characters from all corners of the DC comic book universe to life. Many of these are awesome versions of the heroes, villains, and supporting characters we have loved on the page for decades. A few of them it has even improved.
On the other hand, though, the Arrowverse shows have really dropped the ball with several characters who should have been amazing, but were either underused on TV, taken in a less-than-faithful direction, or just plain badly written.
Sometimes these are lesser known characters who could have been finally given their due on screen, but the opportunity was sadly bungled. Other examples are some of the biggest characters the Arrowverse has at its disposal, yet the writers didn't treat them in the way they deserve.
Some of these characters have appeared in just one or two episodes while others have featured in several seasons, but what they all have in common is that each of them is a great comic book creation who was totally wasted on the small screen.