This one is actually a rather difficult list for me with so many good ones:
1. Poirot, Miss Marple
2. Morse
3. Touch of Frost, Inspector Lynley Mysteries
4. Midsomer Murders, Foyle's War
5. Dalziel & Pascoe, Murder City
6. Murphy's Law
And a few other "who done its" I like, although not true "detective stories" are: The Mrs. Bradley Mysteries, Wire in the Blood, Silent Witness & Waking the Dead
I liked the Lewis Pilot a lot but can't rank it until I watch it this summer