South Korean Teenagers Rank US As Nation’s Third Greatest Enemy | ROK Drop
1. Japan : 44.5%
2. North Korea : 22.1%
3. United States : 19.9%
4. China : 12.8%
5. Russia : 0.6%
As for why China ranks low in the "Greatest enemy ranking", the reason is that Koreans see the PLA as a joke army that could easily be crushed and doesn't present much of a threat in the event of a war, not that Koreans see China as a friendly country. The fact that average Koreans see the almighty PLA as a joke army may stun the Chinese posters here who firmly believe that Koreans suffer from "an inferiority complex" toward China, but that is simply far from the reality. Rather, Korean youth are taught to "look down" on Chinese and consider everything associated with China as automatically inferior.
The Chinese posters can challenge the validity of Korean public perception of the PLA as a "joke army"(Personally I disagree, as China's "human wave attack" can more than make up any deficiencies in quality in a real war) but that's how Koreans see things, valid or not.
This is the same reason Japanese rightwingers spend 24/7 badmouthing Korea and not China, as they see Korean economic and cultural invasion as an existential threat to Japan while China's rise isn't considered much of a threat and any Chinese military attack on the Diaoyutai would easily be repelled(Once again the "PLA is a joke army" perception prevalent in Japan even if the JSDF would disagree). It is not the new carriers and nuclear subs that Japanese rightwingers see as threatening, but the Korean ramen stocked at convenient stores, Korean liquor that's the new fad in Japan(Korea runs a liquor trade surplus against Japan), and the Korean drama on TV that Japanese rightwingers find to be threatening.