The ROKA had ex US M1 57mm AT guns at the beginning of the war. They had limited success against North Korean tanks (all of which were T-34-85's, none were T-34-76's). Many were captured by the North Koreans and used against, and then sometimes recaptured by, the UN forces.
The US Army basically did not use towed anti-tank guns at the time of the Korean War. One exception was the 187th Airborne Regimental Combat Team which used a few T6 90mm AT guns, a limited production weapon from late WWII, but never AFAIK encountered any enemy tanks.
Early Commonwealth units in Korea brought some 17 pounders but hardly used them; Korea wasn't good terrain for heavy AT guns, and after November 1950, the enemy armor threat largely disappeared for the rest of the war. They replaced them with much more portable and all around useful 75mm recoilless rifles in 1951.
The 3.5" (M20) bazooka was rushed to Korea in the early weeks of the war where it had good success, after 2.36" (M9) types had proved deficient v T-34's. 57 and 75mm recoilless rifles were again very useful weapons overall because light and relatively easy to lug up steep Korean hills, but not so useful against T-34's.
The most effective airborne AT weapons proved to be napalm. Rockets were good if they actually hit the tank, but relatively seldom did. Near misses usually stirred up a big cloud of dust around the tank that the pilot interpreted as a 'kill', leading to massive overclaims by rocket equipped fighter bombers of how many tanks they were really killing. The same thing happened in WWII also. Early in Korea US pilots came to believe that their regular 5" HVAR's were bouncing off T-34's and a crash program developed a HEAT rocket, the ATAR, in a few weeks during the summer of 1950. But later tests of HVAR's v captured T-34's showed they were quite effective actually, if they got a solid hit.
As far as UN tanks, ones like M26/46 were most useful when the enemy tanks were around (no Centurions were there yet at that time), but later on the older M4 was the more useful because it could climb hills best.
Joe