Homicide: Life on the Streets is a fantastic series, with three great seasons and exceptional episodes throughout the run. It is also based in the writing of David Simon, who also created The Wire.
Arrested Development is a fantastic surreal comedy that was critically acclaimed but ignored by the general public, so I consider that show under appreciated.
Finally, The Odd Couple, though I would say it’s still appreciated by a great many people, just less appreciated than it deserves to be. It’s a witty and charming series that is still funny 40 years later.