Johnnie Gray deciding to get back his girl and his engine.
Johnnie Gray beating every trap the Union lays for him.
Johnnie Gray finding the General at the Union yard and storming into the cab, retaking his engine at last.
The shot of a locomotive falling through a bridge into a river, the single most expensive shot of the entire silent era.
The cannon sequence, keeping in mind all that was done when computers, let alone CGI, were decades away so everything that's happening is 100% real and had to be timed accordingly. To get one shot, Keaton had to measure out the cannon's powder charge with tweezers.
Johnnie clearing a board off the tracks by hitting it with another board, tiddlywinks style. In his usual fashion, Keaton seems completely unfazed as the huge piece of wood flies past his face.