I’m qualified to answer this since I’ve eaten a lot of instant noodle brands from all three countries.
They do tend to be quite different from each other, I can’t say that one is necessarily better than the other.
But in general
Japanese instant noodles (Nissin, Sapporo Ichiban, Maruchan, etc)
-the Noodles are normally wheat noodles and thin but not as thin as rice noodles.
-Reserved, traditional flavors. There is traditional japanese flavors such as shoyu (soy sauce), shio (salt) and miso, the original ramen flavors. These might be considered quite plain flavors for Koreans and Chinese but they are traditional to Japanese and Japanese do not tend to experiment with any wild or unnecessary flavors, only on what has worked for them (Instant noodles were invented in Japan and has been largely the same for 7 decades in their country).
-there are rarely more than 1 seasoning packet. Again, Japanese noodles keep it simple.
Korean instant noodles (Samyang, Nong Shim, Paldo etc)
-Noodles are alot thicker than Chinese and Japanese noodles. Almost Udon-like.
-They tend to be spicy, like the famous Shin Ramyun or the recently trending Buldak Bokkeumyeon.
-Kimchi flavors are abundant. Some brands even add a packet of kimchi (either dehydrated or moisture sealed) to add.
-Korean noodles also keep the packets rather simple with usually 1–2 seasoning packets but they may add in gochujang or kimchi as I mentioned.
Chinese Instant Noodles (too many brands to list, I like Baijia though)
-Noodles tend to be thinner than both Japanese and Korean noodles. They use vermicelli or rice noodles a lot. Sometimes also thicker fried noodles are used.
-The flavor is varied, some can be spicy (or Mala, based on Sichuan peppers), and some can be sweet or sour or mild. Chinese noodle flavors run the whole gamut.
-The number of packets can be large, sometimes including giant packets of pickled vegetables and lots of packets of soy sauce or chili oil. Chinese noodles definitely give you bang for the buck.
I would also like to throw in Southeast Asian Instant Noodles (Monde Nissin, Mama, Indomie)
-The noodles are usually fried, and quite thin, maybe between Japanese and Chinese noodles in thin-ness.
-The packages are usually smaller than with Chinese/Japanese/Korean noodles. Not sure exactly the reason why, maybe Southeast Asians eat just a small portion at a time?
-Flavor of Southeast Asian noodles varies but they are regional. The famous Indomie Mi Goreng or Mama’s Tom Yum variants taste like the traditional SE Asian dishes they are based on with a hot and sour taste. They are spicy but not as much as either the Chinese Mala noodles or the Korean spicy noodles.
-The seasoning packets tend to include lots of spices and oils, like in the Chinese noodles, but usually do not include the big packets of vegetables.
-These noodles are also usually the cheapest, maybe due to their small size.
Happy instant noodle eating guys!
BTW I do have to give credit to Chinese instant noodles for some innovation. Some noodle bowls / boxes come with a heating packet so you don’t even need to boil water, you could literally just bring this box to a campsite and then take some water from a river and put it in here and start eating hot noodles. That’s just cool.