I voted for the China theater (which should come as a surprise to absolutely no one).
I it was a very colorful with lots of interesting characters. From the initial battles in the north and Epic struggle for Shanghai to the battles of Changsha and the Burma road the war covered a wide variety of climates and areas. Also, with Nationalist and Communist guerrilla running around the rural areas fighting each other as much as the Japanese, Japanese puppet troops and other minor and sundry warlords changing sides all the time in the ever shifting sands of alliances and betrayals, it makes the Vietnam war look like a duck pond.
China was an important ally during WW2 and effectively tied down millions of Japanese troops. Had China not doggedly and stubbornly resisted for the eight years that she did (and endure all the suffering that would entail) all those Japanese troops could have been put to use on other fronts, like India or the Philippines, Okinawa or the Home Islands, all which would have greatly changed the outcome of the war for the allied powers. The fact that the Chinese were able to do this with one of the must corrupt and inefficient military organizations ever seen in history is a testament to hardiness of the peasant soldier and the real skill of commanders in the Chinese army/armies who did manage to achieve the few victories China had during the war. Stillwell (my favorite US commander) showed that with proper training, equipment, support, supplies, and leadership Chinese soldiers could fight just as well as any other in the world.
China’s brutal military culture treated and used it soldiers like the marauding peasant levies of an earlier century. Historically, there had been very little reward given to Chinese troops for victory beyond the opportunity to pillage and no real emotional stake in any cause beyond his immediate unit. Caution and cunning were admired and respect did not come from initiative and dash in the attack, or endurance in defense. Unless victory came quickly they tended to fall back, though even after a headlong retreat in the face of a superior enemy, the long suffering Chinese soldier could be brought back to the fight and stand firm and beat their enemy back again.