Though Kabutops is insanely powerful and way too fast with an SD and rain up (down?), I think that needing these two things, which requires two turns of set-up and a safe switch in between, don't allow it into A-rank. It's not really that bulky and its typing lets it down more than it helps it, and its only real place is on a dedicated rain team. It really doesn't have the move slot to use Rain Dance because it really needs SD, its two STABs, and Aqua Jet.Kabutops is easily one of the best rain sweepers in the tier, +2 stone edge with life orb is capable of OHKOing quite a few bulky water resists, though a number only after stealth rocks, including but not limited to shaymin, kingdra, blastoise, and qwilfish after intimidate. Suicune, and slowbro remain narrow 2HKOs, virizion and empoleon are both OHKO'd or a narrow miss of an OHKO by waterfall in the rain, depending on the spread. Aqua jet KOs 252/120 sableye to avoid priority WoW.
Other than that it OHKOs and outspeeds the entire tier as jolly, and up to base 105 + nature choice scarf users as adamant, without issue using waterfall, and has the defenses to stand up to most priority. Because it has the capacity to sweep the entire tier without ludicrous set up, perhaps it is worthy of A even. It does at least 80% to the best defensive water resists in the tier, that's really not asking for all that much in the way of wearing down if you ask me.
Rocks, rain, and a swords dance can clean sweep a lot of teams, lackcing rocks with a bit of wear on key counters a sweep is also very feasible. Failing that, just in the rain offensive presence is not lacking without a boost, and even out of the rain the strength of aqua jet is real.
Outside of this, it can serve as an spinner and a Stealth Rock setter, but lack of recovery (just like every spinner in UU) and inferior defenses to most other spinners or Stealth Rock users hurts it a lot. It has a niche, yes, but A is too high.