Open Source Content Management Defense: Water Gun with Aqua Tail. Also fast and frequent enough it can force more dodging — or damage.
Counter: Exeggutor with Extrasensory (Legacy: Zen Headbutt) and Solar Beam. (I also like Zapdos, Raikou, and Jolteon because they multitask against Gyarados.)
Defense: Zen Headbutt or Pound with Dazzling Gleam. Dazzling Gleam is harder to dodge than Hyper Beam, making Blissey even more frustrating. But really, any Blissey moveset works on defense.
Counter: Machamp or Hariyama with Counter and Dynamic Punch. Or anything that can do a ton of damage, including Solar Beam, Overheat, or Stone Edge.
Defense: Water Fall with Hydro Pump.
Counter: Zapdos, Raikou, or Jolteon. Take water, add a spark, watch it sizzle.
Defense: Mud Slap with Stone Edge. Still not the fastest but lots of damage, especially against those who don't dodge.
Counter: Gyarados or Vaporeon with Hydro Pump or Exeggutor with anything and Solar Beam. They're plentiful and powerful.
Best Movesets: Mud Shot + Earthquake. Like Ryhdon, only Legendary.
Secondary Movesets: Mud Shot + Solar Beam. Because Solar Beam.
Counter: Gyarados or Vaporeon with Hydro Pump.
Best Movesets: Waterfall + Hydro Pump. The new best Water-type in the game
Counter: Raikou with Thunder Shock + Wild Charge.
Best Movesets: Dragon Tail + Outrage. The new best Dragon-type in the game
Counter: Articuno with Frost Breath + Blizzard or Ice Beam., unless the Raquaza has Ancient Power. If that is so, use Raikou with Thunder Shock + Wild Charge.
Best Movesets: Dragon Breath + Outrage. When a Dragon, go all-in on Dragon.
Counter: Any of the big four Dragons with big Dragon movesets. In a pinch, Tyranitar with Bite + Crunch.