I know that it's hard question to answer, which is why I put it in the form of a question, rather then a poll. There have been so many great ones over the past 20 years. My early favorite was Sgt. Phil Cerreta (Paul Sorvino) and of course Mike Logan (Christopher Noth) made a big (and re-occurring) splash.
Lately I'm very impressed by Cyrus Lupo (Jeremy Sisto, late of Six Feet Under) and (from Law and Order CI) Robert Goren (Vincent D'Onofrio) and very impressed by his partner Alexandra Eames (Kathryn Erbe). Then from Law and Order SVU we have Olivia Benson played by Mariska Hargitay, who won a very deserved Emmy award. I also thought that they never gave enough of a chance to the character to develop of Joe Fontana (Dennis Farina). Then there is Zach Nichols, played by Jeff Goldblum. He has a huge future in the franchise if he doesn't decide to go back to movies.
Now it seems that there will be a new entry to the series called Law and Order Los Angeles that has just gone into casting. Let's hope that Dick Wolf puts as much care into casting it as he has (most of the time anyway) casting all of the other shows.
Still, for me anyway, it doesn't matter. For me the iconic detective of all of the franchise (as opposed to the iconic DA (Sam Waterston as Jack McCoy) is and always will be in my heart Jerry Orbach, playing the quintessential rumpled and world weary detective Lennie Brisco. To me there was always something about the way he carried himself and something about the look of his eyes, that spelled out a detective who had been through everything and had seen it all. He (with Jack McCoy as a close second) is the very face of the Law and Order franchise.
So, what do you think? Who is your favorite Law and Order (all of the franchise) detective and why?