Good question, well framed and I'll stick to sets I actually have rather than figures I've seen on PSR that look far better but I don't have any period interest in (cough-WSS-cough).

The recent Boer infantry set has loads of great sculpts with so many versatile uses with only a couple of bum notes. A theme of irregulars cautiously advancing and firing. Looking at PSR and the figures themselves it comes down to the 2 figures advancing, head down, on the bottom row and my choice is the middle one cycling his rifle-bolt.

http://www.plasticsoldierreview.com/Review.aspx?id=2708


(Of course I won't mention some other favourites from the War of 1812 American Militia and Tecumseh sets as well as WW1 Russian Winter Uniform and RCW Red Army Infantry both winter and summer)

First from the right.

edit: Set is WWI 9.2-inch British Siege Howitzer MK I