I don't think KISS' rhythm guitars (in any era) border on the insanely difficult in terms of the technical execution. But nailing the proper feel and locking into the groove can always be a challenge. As far as difficulty, I'd go with the main riff to "Parasite" and probably something like the main riff to "Young And Wasted," given they are both based on strong 16th-note feels.
Here's my take on guitar solos:
AF: Ace had such a knack for writing cool guitar solos. And the best part is that most of his better leads are not impossible to play. But again, getting his feel and groove down is a whole other conversation. I'd probably go with "Parasite" or "100,000 Years" -- both are technically challenging (as far as Ace's solos go). Or how about the solo to "Almost Human"?
VV: I'm going with "Fits Like A Glove." The first portion is essentially based in the trusty A minor pentatonic box at the 17th fret but Vinnie's attack, phrasing and whammy work make it difficult to emulate. Ditto on the second-half of the solo. Add everything up here and it's surely a challenge to get in the ballpark of what Vinnie's doing. Runner-up: "Not For The Innocent" solo.
MSJ: Been listening a lot to "Animalize" lately and trying to break down some of Mark's solos (thanks to Riffstation). I think "I've Had Enough" is among the more challenging. There are a lots of notes at an aggressive tempo; and his phrases range from in the pocket (that introductory 16th-note sequential motif) to peculiar (notably his whammy work and that melodic climb he does as the solo climaxes and gives way back into Paul's vocal). Judging from Bruce's take on the "I've Had Enough" solo (especially his rough attempt from the Stockholm 10/26/84 gig), I think he might agree. In truth, I think all of Mark's solos on "Animalize" pose a challenge.
BK: "Hate," just because of having to emulate the phrasing via whammy pedal.
TT: I haven't studied his playing enough to list his most difficult solo. But I think he did a nice job on "Modern Day Delilah."