It's gonna be a high diff fight either way. I remember Hisoka rating Illumi as 95 (out of 100?) which was higher than the 3 zodiacs the highest of which was rated as 90. From what we've seen. Feitan might be the 2nd strongest of the PT. But Illumi is in the same league as Hisoka it seems, and Hisoka is fixated only on Chrollo from PT. Since we haven't seen much of Illumi can't really say. But if I had to choose I'd say Illumi, since Illumi should be one of Killua's main adversaries to defeat and surpass, much like with Gon and Hisoka.
All Illumi has to do is pierce Feitan once, not too mention Hisoka was only focused on fighting Chrollo, not anyone else, which goes too show his only interest in the Troupe was Chrollo. Feitan is strong but Illumi is higher ranked than 3 zodiacs and is more or less equal to Hisoka. Illumi definitely wins this
illumi. illumi is positive he can kill hisoka based on the fact that he threatened to do it and he's a firm believer that you should never fight anyone who is stronger than you. it wouldn't be an easy fight but illumi walks away from this in the end
Manipulators are hard to deal with, its game over if they get 1 hit on you and that makes them a OP class. Feitan's rising sun is pretty useless here, because Illumi will try to control him instead of physically harming him. He has to finish Illumi quickly and if he couldn't finish Zazan fast... Yeah.
Feitan wins neg diff! Illumi has no feats and if sticking a needle into you is his only attack then he only wins zero out of ten times against Feitan. Feitan's speed is hax and his hatsu is hax as well, and that was only one of his Hatsu as Phinks clearly stated there are other forms and versions of his power. But Feitan from what I've seen from both of them doesn't even need his Hatsu to take this guy out, speed blitz and sword slash is all that's needed. The only way Illumi could win is if he has something similar to Shalnark's autopilot mode. If he does, then I have no idea who wins. But if all he can do is throw needles he gets neg diffed.
Hisoka is hyped to be as powerful as Kuroro, and Illumi is portrayed to be as powerful as Hisoka, or at least rivaling him. Kuroro is the strongest spider, but you can go ahead and say Feitan is the second strongest, but I give Illumi the win.