The philosophy of Bill Polian and the front office is to have a revolving door at linebacker. Because of the existing salary cap, emphasis and investment in certain key positions is vital to maintaining the core of your team. For a Tampa-2 style defense, safety and defensive line are much more important than linebacker. Thus, players like Bob Sanders, Dwight Freeney, and even Anthony McFarland were considerably invested in (aka, paid more money) over Colts linebackers.
Still, despite the revolving door, some very impressive linebackers have made their way through the Colts over the last decade. Many of these players played well in Indy, and then went on to play even better at other destinations. However, the focus here is on players who played well while in Indy. This includes a player like Marcus Washington, who went on to play in Pro Bowls for the Redskins. However, before his Redskins days, Washington was a vital player in Indy's AFC Championship run in 2003. Polian still refers to Washington, and later David Thornton, as linebackers that "got away."
Then there are the "solid" players; guys who gave consistent effort and dedication for long stretches. Players like Gary Brackett, Rob Morris, and Thornton fit this. Big play guys, like Cato June, were less consistent, but were often part of several HUGE plays that helped win football games for the Colts. I've also included Clint Session because he has been very impressive in his three years, moving from back-up to starter very quickly.
Whatever your criteria, the voting is below. Use the comments to write-in anyone who think was slighted.