Although this is a straightforward question, the answer is not so simple. Both are excellent fighters, and both share exception Nen abilities. But the victor dramatically depends on the area they are fighting.  Hisoka will probably win in a one-on-one fight due to his incredible speed and strength. However, if they fought in a city environment, Illumi would have the upper hand as he could control many people. And would probably win due to the endless supply of human fodder.