Sometimes a book series starts with a bang, ends in a whimper and dodders around in the interim, but readers are loyal and will wait out a few weak entries if they like the characters and setting and mood. Looking at my reading journal for 2011, I noticed that I read several continuations of mystery series that have mostly stayed strong from the first book. For "cozy" mystery fans, I recommend Susan Wittig Albert's China Bayles series, set in Texas and featuring herb-growing sleuth China Bayles. In Mourning Gloria (2011), the series is at top form. For fans of the British cozy, try Alan Bradley's Flavia de Luce series. This series is sometimes classified for teen readers, but it is suitable for all ages. Flavia is really a pretty obnoxious pre-teen sleuth, but lovable in her own quirky way. Speaking of quirky, try Christopher Fowler's Bryant and May series. Set in London, two old friends in the police "Peculiar Crimes Unit" solve mysteries with their unconventional methods and treasure trove of cerebral trivia about London's history. Chris Evan's Good Thief's Guide to (insert city name here) series never fails to entertain. His good thief/narrator's adventures remind me of the caper movies so popular in the 1950's and '60's. Topkapi, Charade etc.
On the darker side of mystery series, I can always count on Jacqueline Winspeare's Maisie Dobbs series and Louise Penny's Inspector Armand Gamache series.