In rain Kingdra is one of the most threatening Pokemon.
Waterfall + Outrage combo is only resisted by Shedinja and Empoleon, they're both physical attacks (boosted by Dragon Dance) with 180 power after STAB (Waterfall gets an extra 1.5x boost from Rain, after a single Dragon Dance you're looking at Kingdra's attack stat being boosted from 317 to 475).
Rain doubles its speed so minimum speed (no EVs, no IVs, Neutral Nature) will be 350 (the same as a Gengar with max EVs, Max IVs, and a Positive Speed Nature, after a Dragon Dance this will be 525, which will out speed a max speed Deoxys Speed).
Special versions have a larger immediate danger due to the higher power of Surf, Draco Meteor/ Dragon Pulse and access to Ice Beam however don't have any options to increase their special attack power (par Surf/ Hydro Pump from rain).
Kingdra has some support options in Yawn and Toxic but not much else.
Kingdra's base stats are: 75/ 95/ 95/ 95/ 95/ 85 meaning the only stat that is below average is HP, its speed is average and everything else is above average.
To put this in to perspective, Mew, Celebi and the like have all of there base stats at 100.
After all of that I still haven't mentioned Item (possibly Damp Rock, Leftovers or Life Orb), or the other 258 EVs.