Yeah, it depends on the level, as well as the pokemon's EVs, IVs, and if you're talking about the attack when you say which is "stronger". But pachirisu has higher base stats than pikachu, so I guess pachirisu is stronger. Another factor could be the moves they can use. Or if you're talking about the anime.
pikachu ofcaorse.you are talking about ash's pikachu and dawn's pachiriso, arent you.pikachu is what?level 100 by now and then theres the newby trainer with a weak pokemon.the answer is clear