Shilo was a two day battle with 27,000 fatalities for both sides combined.
The first day of the Somme was pretty nasty by all accounts. 57,000 British casualties of which 20,000 were fatalities.
The assault of Badajoz by the British took 40 attempts and cost 3500 men dead in one morning. The 48th Regiment only had 35 men and a couple of officers left. The victorious British then went beserk killing anybody in sight. I'd say that was pretty 'intense'.
But to be honest, the pre-gun powder, hand to hand combat of the dark and middle ages would probably be the most intense fights you could find. But I think most battles anywhere at anytime would be pretty intense if you are actually on the sharp end.