Question: is illumi stronger than hisoka?
Answer: It's a bit more complex than that. In HXH, with proficient nen users, the outcome of a battle is not as clear as the one of another battle manga. In my opinion, Hisoka is slightly stronger (as he gave Illumi a rate of 95 with himself considered as 100) but I don't think he would win 10 battles out of 10 against Illumi.