This is going to be tough! Although I read a lot of mystery novels from the library, there aren't all that many that I would call favorites. Let me see what I can come up with...
1) Gaudy Night by Dorothy Sayers (and everything else by her, also!) Although Peter is a terrible snot in the early books, Harriet is the perfect match for him and I love watching their courtship progress over the series.
2) I am Half-Sick of Shadows by Alan Bradley (and the rest of the series, although I think this is the best). Flavia DeLuce is a marvelous narrator; think 1950's Harriet the Spy in England with a lot more snark, and a bent for chemistry.
3) Guards! Guards! by Terry Pratchett (and the rest of the Discworld stories about Commander Vimes and his merry crew.) These are fantasy - mysteries, so not traditional at all, but marvelously well done.
4) The Dead of Winter by Rennie Airth. I don't like the sequels as much as this one, which is the the start of the series. It's set in England, winter of 1944...
5) A Talent for War by Jack McDevitt. This is a perfect example of the science fiction - mystery, with a little archeology thrown in.
6) Brat Farrar by Josephine Tey (and everything else she wrote, too!) This is a wonderful story of murder, mistaken identity, love, and double-crosses.
7) The Skull Mantra by Elliot Pattison. I don't love the sequels (in fact, I barely like them) but this book is a marvelous merging of locale (Tibet) and murder mystery.
8) The Case of William Smith by Patricia Wentworth (and many of her others that star the marvelous Miss Silver. Other favorites include The Clock Strikes Twelve and Through the Wall.)
9) Rivers of London (also known as Midnight Riot in the USA) by Ben Aaronovitch (and sequels). This is modern police procedural meets magic realism and the results are magical.
10) Glass Houses by Jane Haddam, and many of the others in her long-running series about Gregor Demarkian.