vo nguyen giap in my opinion is the most underrated. He beat the Japanese, French, USA, and Chinese with a military that started with almost nothing, and had to fight for decades straight.
The americans may have won some ground battles, but they lost the war. Especially impressive is the fact that the americans dropped more ordnance on vietnam than they did on Germany and Japan in WW2 combined, and the vietnamese were working with complete air inferiority. America had pretty much the most powerful military in history at that point, up against a bunch of jungle farmers, in a country only 10 miles wide. With constant ambushes and sabotage, Giap left a whole generation of GIs with PTSD. Plus, this was the first war televised in real time. American public opinion turned against it with the relentless guerilla warfare, and the American politicians gave up and left.
I would also give a shout out to alexander suvorov and paul emil von lettow-vorbeck. Without Paul, we wouldn't have guerrilla warfare today as we know it