Kishimoto told us they were close enough in strength 2x times, both in quick succession. When first meeting one another and a threat is made, Itachi doesn't hesitate to turn on the Mangkeyou Sharingan implying anything less is not enough for Hoshigaki Kisame [
1]. Then when Kisame has drifted off from this mortal coil, Yamato respectfully says he's an Akatsuki powerful enough to be paired with Itachi [
2]. Yamato has so far seen Kakuzu in action so this is high praise that deliberately sets Kisame apart from the rest of the Akatsuki.
In terms of powers, ninjutsu will not work—not even katon or Amaterasu. Despite its failings, Samehada can absorb heat based ninjutsu but has a dislike of it. However, Kisame is capable of forcing Samehada to do as he pleases despite itself [
3 —>
4]. Likewise at any point that Itachi decides to pull out a V3 Susano'o, the minimal requirement needed for Yata Mirror & Totsuka Blade [
5], Kisame will have created a wide enough sphere of water and has good odds of escaping any attack or even when unfused [
6]. When fused with Samehada, Kisame is faster than a perfect Jinchuuriki in water [
7]. Don't forget that Naruto, no documented swimming feats, was blitzing 2 Tomoe Sasuke when influenced by the Kyuubi, in water during VoTe1 [
8].
So in short; Amaterasu gets absorbed and the threats of a V3 Susano'o and the Totsuka blade will never come to fruition because by that point in time, Kisame has a lot of liquid & water to move around in.
So Itachi needs to hit Kisame with Tsukuyomi before Kisame fuses with Samehada—because Kisame no longer requires eyesight to hunt when fused [
9]. The principles of disrupting regular genjutsu are as follows; irregular chakra flows [
10]. Kisame, via Samehada absorbing and sending back chakra can create an irregular flow to avoid regular genjutsu in the same way a Bijuu & Jinchuuriki are immune to regular genjutsu. It's really only Tsukuyomi that gets Kisame.
There's other factors like battle IQ; Which Itachi exceeds Kisame but Kisame is no slouch either, stamina & speed. Itachi has a massive speed advantage which cannot be ignored and he's the more intelligent of the two, so I give it to Itachi 6.5/10 times but kisame has nearly everything required to take down Itachi.
He fight against Bee was pleasant
