It's hard to pick one battle scene...
"Big" battles - 19th Century- a choice between the thunderous opening barrage of Pickett's charge in "Gettysburg", and the climax of the charge.
"small" battles - 19th century - the final Zulu assault on Rourke's Drift from "Zulu".
Infantry combat - WW1 - "The Lost Battalion" still holds a small edge over "Alls Quiet on the Western Front" in my book.
Infantry combat - WW2 - I still like the German early morning attack from "Battleground". It captures the uncertainty and surprise of a small unit action. Of course "Saving Private Ryan" has several good scenes. I really enjoy the battle for control of the bridge, while the invaision sequence is the most historically accurate.
Infantry combat - Korea- the struggle of Gregory Peck's company in "Pork Chop Hill"
Infantry combat - Vietnam - the climactic action of "We Were Soldiers".
Infantry combat - modern - the Rangers struggle in "Blackhawk Down".
tank battles - small - the scene in "The Tanks Are Coming" when the Shermans meet their first "Panther" (actually an M36 TD). 2nd choice is the attack on the 88 at the river crossing.
tank battles - large- who can resist the tank charge at the climax of "The Battle of the Bulge". Hoaky as can be, but all those "Shermans" (M24 Chaffees) versus the "King Tigers" (M47s) maneuvering, shooting and mixing. A good child-like view of armored combat, but a guilty pleasure to watch.
air battles - WW1- "The Blue Max" has several
air battles - WW2 - "Battle of Britain" has several good scenes, but the classic "Twelve O'Clock High captures the tension of "sweating out" a battle from the ground.
air battles - jet - how can you beat "Top Gun" for pure jet fighter thrills?
naval battles - sail - a toss up between "Master and Commander" and "Captain Horatio Hornblower" (the Gregory Peck version)
naval battles - WW2 surface - I still like the climactic surface battle from "In Harms Way". Not historically accurate, but the models are quite good.
naval battles - WW2 submarine - "Das Boot" was good, but the running battle of "The Enemy Below" is still my favorite.
Those are some that I can pick off the top of my head. All are just personal favorites over the years.
Gary