Merri Brody - NCIS: New Orleans
Brody, we hardly knew ye. For two seasons. Brody served with Dwayne Pride's team, without a whole lot of character development. Then she got romantically involved with the wrong guy and quit before NCIS: New Orleans: Season 3. Why? One rumor is that Zoe McLellan left the series for personal reasons. Another was that Brody was written out for creative reasons. Make your own guess.
Tammy Gregorio - NCIS: New Orleans
Brody's sudden exit left an opening which was filled by Gregorio. Following Brody's Midwestern sweetness, Gregorio's New Yawk attitude has been a tough sell. Fortunately, the former FBI agent received a well-developed back story, and she's softened over the past two seasons. Now what happens when Shalita Grant's Percy leaves after this season? Fortunately, Christopher LaSalle is a constant.
Lance Sweets - Bones
The cerebral Lance Sweets earned his place among the FBI/Jeffersonian crew, despite skepticism from both Brennan and Booth. Fans also enjoyed his on-again, off-again relationship with "squintern" Daisy. But after nine seasons, John Francis Daley's burgeoning directing career led to Sweets being killed off the series.
James Aubry - Bones
Sweets' death left an opening for a new sidekick for Booth for the series' final four seasons. Like Sweets, Aubrey also became involved with a "squintern." He also got derided for his horrible eating habits. Later in the run, it was revealed that Aubrey's father was a fugitive white-collar criminal. Aubrey wasn't an offensive character. He just wasn't Sweets.
Gil Grissom - CSI
For nine seasons, Gil Grissom was the quirky voice of wisdom for the Las Vegas CSI unit. Grissom provided the vibe for the series, and made science geekiness cool, as forensics programs popped up all over the country. When William Peterson left to take on other roles, it set the cast carousel in motion on the venerable procedural.
D.B. Russell - CSI
D.B Russell became the graveyard shift supervisor at the start of CSI Season 12 and stayed for the final four seasons. He even jumped over to CSI: Cyber Season 2, the final gasp of the franchise. You can never go wrong casting Ted Danson (with Becker the notable exception). His Russell was a calm space in a crazy world, that of Las Vegas after dark.