Thought of a few of my own. Probably want the sound down some, if you're in work not doing work.
This one sums up a big theme of the show, and gives a good early look at Lester Freamon, Smartest Cop In The Room:
[youtube]http://www.youtube.com/watch?v=a_eH0Kt4DNA[/youtube]
The spot where Stringer Bell finally cracks, and one of those scenes that reminds me of how Wood Harris often got overlooked on the show. Plus, we get a brief glimpse into Avon's DVD collection:
[youtube]http://www.youtube.com/watch?v=WkVohPC_YpU[/youtube]
Two more where I haven't seen clips around:
Season 4, Royce shaving and getting himself into "fighting shape" for the election battle with Carcetti. Probably the first time they gave us a look at what Royce must have been like once and a warning of what Carcetti was going to become.
Early season 2, McNulty happily working away at a math problem with some graph paper in the harbor patrol offices. Maybe the best example of what drives McNulty, that combination of wanting to prove how smart he is, wanting to actually solve crimes and wanting to screw over as many people as he can in the process of doing both.