Janet Evanovich's Plum series has definitely waned. 1 to 12 were good and then it went downhill. 13 and 14 read almost as if someone else wrote them, but 15 sounded more like Janet. Although the series should be classed as humour (young teen humour at that) rather than 'thriller/mystery'.
I prefer the in-between books with Diesel, although starting next year, he's getting his own series.
J.D. Robb's series is just getting better and better, I don't think I'll ever tire of Eve and Roarke.
Sherrilyn Kenyon is another author whose series goes from strength to strength. Her Dark Hunter series continues to be a good read.
J.R. Ward is another who manages to keep a series going. Even though book 6 was a little weak in my opinion, book 7 'Lover Avenged' showed that Ward is back on form.
Just my two cents worth.