Kabutops@Damp Rock
Adamant Nature
Swift Swim
-Rain Dance
-Waterfall
-Stone Edge/Rock Slide
-Dig/Brick Break
Rain Dance for the Swift Swim speed boost, as well as a double STAB for Waterfall
Waterfall primary attack, base power of 180 after rain as well as a 20% flinch chance
Stone Edge will have a power of 150 after STAB, and also be less accurate than Waterfall... has a high critical ratio. Rock Slide on the other hand, will have a power of 107 after STAB, but you'll maintain and even improve your flinch chance when you have to use this over Waterfall. I'm a tad biased towards the flinch chance and extra accuracy, but I'd pick Rock Slide.
Last attack is soley for pokemon resistant to both of your other attacks... namely Empoleon and Quagsire, though I'm willing to be there are at least one or two more...
Dig will make your opponent switch out [especially Empoleon]... flying types are weak to your rock attack, though Skarmory may give you a bit of trouble on defense, I don't think it can do any serious damage to you. If it whirlwinds, you don't exactly lose your speed boost if you come back out before the extended rain is done. Carnevine and Bronzong are the only levitating threats... and with any luck you can make Bronzong flinch and score another hit.
Brick Break offers the same Empoleon coverage, with a slight lack in power as opposed to dig.. it also won't be making opponent's switch on you [if you weren't assured by what I said above]. The main reason I'd choose this over dig.. Dig wastes one of your Rain turns and leaves you open to Earthquake if it's seen coming.
I'd pick Dig over brick break if you had Stealth Rock support, otherwise I'm about fifty fifty on them.. Dig forces your opponent to make a tough choice between switching to something that's likely weak to one of your other attacks, and be slower than Kabutops on the draw. But it still uses an extra rain dance turn, and leaves you open to a souped up Earthquake.
EVs:
252 Attack
180 Speed [+4 for every point below 31 your speed IV is]
76 Defense [or whatever is left after speed]
OR
252 Attack
132 Speed [+4 for every point below 31 your speed IV is]
124 Defense [or whatever is left after speed]
The first spread will allow you to outspeed everything shy of Ninjask/Deoxys after the rain, and the second spread lowers it so that you only outspeed up to base 130, which is everything except Ninjask/Deoxys/Electrode... Electrode actually poses a threat to Kabutops, not that you need me to say that, but it really isn't that commonly seen.. the risk is up to you.
Kabutops' defense will allow it to switch into a physical pokemon and get off rain dance while taking an attack... so long as it isn't leaf blade, Swords Danced, or Super Effective STAB, Kabutops can probably take the hit. I do believe it can survive most pokemon's non-STAB'd Brick Break.