Here are my nominees for the best moments...they are heavy on 3 and 4 but what can I say, those are my favorites. I have not bothered with any from 5 yet, as I know I'll need more distance from it before I can make any judgments. Most of these should be self-explanatory (at least in terms of what scene it is) and I'll defend any of them if asked.
-Snoop learning about the nail gun in 4
-Colvin's paper bag speech
-Freamon and Bunk discovering Marlo's vacants secret
-The opening scene of the series
-Omar and Bunk's meeting on the bench in 3
-Sobotka's argument with his lobbyist ("We used to make shit in this country")
-Marlo's encounter with the store security guard ("You want it to be one way")
-The last meeting between Avon and Stringer "We don't need to dream no more."
-The alleyway stand-off between Omar and Brother Mouzone
-The interaction between Bubbles and Landsman in 4 (it's actually two scenes but they're so goddammed good)
-Brother Mouzone's interaction with Cheese in 2 (left it out cause it's not terribly meaningful, just funny/badass)
-Colvin's speech about police work and what it's become to Carver in 3...left it out cause it's just him ranting again and the paper bag speech is better
-Prez and Freamon's "All the pieces matter" conversation for obvious reasons, I left it out cause it's basically just that line.
-The infamous "fuck" scene...cause that's just too easy.
-Freamon and Bunk talking to all the apparently non-English speaking sailors from The Atlantic Light
-The death of Wallace...cause that also seemed just too easy.
-Slim Charles and Avon in the finale of 3..."If it's a lie, then we fight on that lie."