This is a tough one.
For me the main appeal of the series is in its tight shooting, macabre style, and epic action.
In this regard, 2 and 3 are closely matched for the top spot; both have some truly fantastic setpieces, as well as razor sharp gameplay. 3 is perhaps a little more mechanically polished and packs an extra graphical oomph, while I feel like two probably has slightly more memorable highlights.