Seeing as how no one else ventured an answer despite being given the chance then I shall go ahead anyway. My expertise as my flair denotes is for the British Forces, thus I was hoping for someone with a greater expertise on the overall quality of the IJA could do this comparison. Since it doesn't seem to be the case I shall try my best but my comparison is better suited to the Japanese Troops against the Commonwealth Forces there.

The Japanese order of battle of the Malayan Campaign included the 5th Infantry Division, 18th Infantry Division and the Imperial Guards Division along with all their respective support units. General Yamashita also had the option to bring another Infantry Division with him but elected not to do so in order to ease the burden on his supply lines. The Japanese troops assigned to the campaign were by no means crack units that received special training such as airborne forces or considered elite or above average formations like the American Rangers or British Guards. The 5th and 18th Infantry Divisions were still formidable as veteran units from the China war having racked up considerable battle experience. The Imperial Guards division on the other hand despite their title were actually an untested unit used largely prior to this for parade and ceremonial duties. The Imperial Guards were not distinguished by any special training but rather by the recruits they chose which were Japanese of larger stature which would look better on ceremonial duties and such. They largely proved themselves during the campaign by performing up to expectations despite this being their first real combat action.

The success of the Japanese Campaign there was definitely in part due to their troops, although I would attribute it primarily to Yamashita's exceptional generalship. The Japanese forces had the distinct advantage of being homogeneous in culture thus reducing operational friction and logistical problems. The Commonwealth forces on the other hand came from a diverse background with the Indian troops alone having an extensive diversity in religion complicating their dietary requirements. While the Japanese logistic train was streamlined in the sense that all their troops ate the same dried rice rations. In addition, British sources state that there was great operation friction between Indian Army units and in particular the Australians who were in contempt of the former. Multiple primary sources from Colin Smith's Singapore Burning recount that Australian troops moving to relieve the III Indian Corps that was retreated after being battered by the Japanese were rowdy when they cross paths with the withdrawing Indians. Accounts state that the Australians being rowdy and excited for battle attempted to shoot out the headlights of the trucks carrying the retreating Indians for fun as well as shouting derogatory slurs despite being on the same side. The Indian Army units also consisted of Hindus, Muslims and Skihs which created massive cultural tensions when units were amalgamated later in the campaign to form multi-ethnic ones reducing their cohesion significantly. The British army units aside from the Agryll and Sutherland Highlanders, East Surreys and a few other regular British Battalions were largely newly conscripted or untested sepoy units in the Indian Army and a mix of new conscripts and reservist in the Australian Division. The Japanese while not being elite units when compared to quality of forces throughout the war were most definitely superior to the largely second-rate troops stationed in Malaya.

The Japanese support units such as the Combat Engineers, Artillery and Signals were also competent veterans but nothing truly exceptional. The combat engineers repaired bridges exceptionally fast during the campaign which sustained the rate of the Japanese advance. However this was not really due to any exceptional skills on their part but rather by the mistake of the Civil Colonial Administration that had stockpiled building materials near many bridges in the peninsular without informing the military in the hopes to speed up a future British counter attack. This ultimately backfired on them with the Japanese engineers using the readily available materials to repair demolished bridges in as little as 8 hours. The Japanese artillery was in fact inferior to the British but this was more due to the disparity in their equipment with the 25-pounder being superior to the Type 91 10cm Howitzer. The Royal Artillery was actually the only branch of the British Army that could be considered to be superior in quality to their Japanese counterpart during the Malayan Campaign as seen at the Battle of Kampar where they were used effectively to stall the advance. However as the terrain limited the use of artillery due to the poor visibility in the jungle and the close proximity that engagements normally occurred at, this superiority could not be effectively leveraged.

In conclusion it was the inferiority of the British forces and not any particular outstanding superiority of the Japanese forces that caused the Malayan Campaign to be such a success. More importantly I think is the generalship and strategies used by both sides that were really the deciding factors in the Campaign as opposed to just quality of forces. For more on that you can look at my earlier post here regarding the Malayan Campaign.

Sources:

Singapore Burning: Surrender and Heroism in WWII - Colin Smith

The Defense and Fall of Singapore - Brain P. Farrell