Rohirrim for emotional powerhouse & eternal cinematic glory.
But Waterloo is really the only correct answer for best overall cavalry charge in movies. There'll probably never be a scene like that again now that we have CGI. Even that short clip linked in this thread is jaw dropping.
Lawrence of Arabia's is pretty crazy because it's all real but in terms of how awesome it looked and what it meant to the story, I gotta go with Game of Throne's Battle of the Bastards.
