Both the great Devraj Indra and Kartikeya ji are very powerful warriors. But Kartikeya ji is more powerful than the great Devraj Indra.
Kartikeya ji is the son of Mahadeva and Mata Parvati and is the commander-in-chief of Gods.
In Mahabharata , there is a mention of battle between the great Devraj Indra and Kartikeya ji, which was easily won by Kartikeya ji.
When he was thus forsaken by the gods, Sakra hurled his thunder-bolt at Skanda. It pierced him on the right side; and, O great king, it passed through the body of that high-souled being. And from being struck with the thunder-bolt, there arose from Skanda's body another being--a youth with a club in hand, and adorned with a celestial amulet. And because he was born on account of the piercing of the thunder-bolt, he was named Visakha. And Indra, when he beheld that another person looking like the fierce destroying Fire-god had come into being was frightened out of his wits and besought the protection of Skanda, with the palms of his hands joined together (as a mark of respect). And that excellent being Skanda, bade him renounce all fear, with his arm. The gods were then transported with joy, and their hands too struck up.”
Source : KMG Mahabharata Vana Parva Markandeya Samasya Parva Section CCXXVI
Even the great Devraj Indra accepted his defeat and asked Kartikeya ji to become the new Indra.
Sakra replied, 'Thy prowess is unrivalled, O hero, do thou therefore vanquish the enemies of the gods. People have been struck with wonder at thy prowess. More specially as I have been bereft of my prowess, and defeated by thee, now if I were to act as Indra, I should not command the respect of all creatures, and they would be busy in bringing about dissensions between us; and then, my lord, they would become the partisans of one or other of us. And when they formed themselves into two distinct factions, war as before would be the result of that defection. And in that war, thou wouldst undoubtedly defeat me without difficulty and thyself become the lord of all worlds.'
Source : KMG Mahabharata Vana Parva Markandeya Samasya Parva Section CCXXVIII.
Kartikeya ji refused the offer out of his respect to the great Devraj Indra, the king of Gods.
Sakra said, 'O mighty being, do thou make us happy, by becoming our lord. Excellent being, thou art worthy of the honour; therefore shall we anoint thee this very day.'
Skanda replied, 'Do thou continue to rule the three worlds with self-possession, and with thy heart bent on conquest. I shall remain thy humble servant. I covet not thy sovereignty.'
Source : KMG Mahabharata Vana Parva Markandeya Samasya Parva Section CCXXVIII.
The great Devraj Indra is a very powerful warrior but Kartikeya ji, the son of Mahadeva and Mata Parvati is easily more powerful than him.