There is no way to know with objective certainty which big cat would win in a clash between any two individuals of the same or different species, including leopards and lions. Lions are larger and stronger than leopards, but leopards are noted for their sudden bursts of ferocity and for their propensity for overwhelmingly savage assaults on animals entering their territories.
The lion's chief advantage, with the exception of physical size and strength, is that if the lion is a male it has a mane to protect its neck. Most big cats kill by going for the throat and ripping or tearing, but the thick hair of the mane confounds this strategy.
Leopards are smaller than lions but are extremely powerful for their size and are experienced ambush hunters, often dropping from trees onto prey in order to take it by surprise. Leopards are also solitary hunters and must fight alone to protect their territory from other leopards, while lions are used to fighting in groups.
Leopards are significantly lighter than lions, weighing in at under 200 lbs whereas a full grown male lion may tip the scales at over 400 lbs. This disparity makes it almost impossible for a leopard to physically overpower a lion in direct confrontation.