Heh. I could go on for days here. I've read a lot of detective novels, and it's one of my favorite genres in general. Having just one fave seems impossible.
Poirot and Holmes remain two of my faves, the classic examples of quirky and brilliant men who could make intuitive leaps no one else could.
My other favorite golden age detective is John Dickson Carr's Dr. Gideon Fell. Carr was a specialist in bizarre locked room situations, and Fell was a fantastic detective who always put together a solution.
Moving forward, Hammett's The Continental Op is my favorite of the classic hardboiled bunch, and Red Harvest remains such an important book.
After that, Rex Stout's Nero Wolfe and Ross Macdonald's Lew Archer are probably my favorite midcentury detectives, both evolving the form dramatically in different directions.
Then there's John D MacDonald's Travis McGee, the classic '60s/'70s outsider, a guy living on a boat and just enjoying his life until he gets pulled into a new case. Meanwhile, Gregory McDonald's Fletch gave us the gifted amateur detective who just keeps stumbling into new cases.
(I have no idea why so many variants of "MacDonald" end up writing detective books, though "Ross MacDonald" was a pseudonym.)
Then the '80s gave us two great hardboiled female detectives (and with all due respect to Miss Marple and Amanda Cross's Kate Fansler, these both surpassed all previous ones), in Paretsky's V.I. Warshawski and Grafton's Kinsey Millhone. Millhone's probably my favorite late-twentieth-century detective.
This century, I'm huge fan of Laura Lippman's Tess Monaghan (a hardboiled former journalist), Lee Child's Jack Reacher (a taciturn former military policeman), and Louise Penny's Inspector Gamache (a police detective from Quebec who takes a compassionate and unusual approach to his cases, many of which end up in the same small arts community).