Level 100 Statistics (see level 5, 50, 100)
Huntail is another "decent yet other things do it better" Pokémon, so only use it on UU teams. If it were Water / Dark, maybe it would be used more, but Gorebyss does EXACTLY what Huntail can do but better. But in UU play, not many things can wall Rain Dance sweepers, so you can sweep even with bad Special Attack compared to others, if for instance, you stack your team with multiple Pokémon to take advantage of the rain. Jörmungandr, Slime Eel, Maria, whatever you call it, Huntail is cool.
Before using Huntail, the first question you should ask to yourself is WHY do you even want to use it? Yeah, you might use it in a theme team, or it might be your favorite Pokémon, but just go with Gorebyss or Omastar, or even Kingdra. Huntail has 94 base Special Attack, which is pretty nice (except it's worse than the three other Rain Dancers mentioned before) so Rain Dance and Surf things away, or since Huntail brings Blissey, Suicune, etc., switch to their counters, so you can sweep with a few good predictions.
Baton Passer, decent as it can deal with Skarmory, but Defense isn't the best stat to pass. But you can always Snatch from another Baton Passer (through they are not that common) and pass it yourself.
Huntail's physical movepool is horrible. Hidden Power Ground for the inevitable Electric-type switch in. Rock Tomb 2HKOs Zapdos, and, thanks to the Speed drop (which is 100% with Rock Tomb), you are then faster next turn and can OHKO. However, if Zapdos has 304 Speed, you tie after the Speed drop, and any higher and you can't win. You could run Jolly to beat it 100% (220 Speed is needed to this, or 240 EVs with Jolly), but then you don't always 2HKO, unless it's in a Sandstorm. Odds are, people will either not notice or ignore that message saying that Zapdos's Speed fell, which works to your advantage. Watch out for that 80% accuracy, though. Body Slam for paralysis, Sleep Talk to absorb sleep.
Screech triggers some switches and maybe can be used if you're running the physical set without Choice Band. Confuse Ray is generally annoying and forces a few switches.
Just go with the EVs listed in UU for the first set. In OU, max out Speed and Special Attack. For the second set, max HP and Attack if you drop Rock Tomb for Ice Beam.
Checks and Counters
Any special wall, especially Water-types, stop the special set. The physical set requires prediction like other Choice Band sets and is slow. Baton Passer has troubles with Suicune and again, Defense is a bad stat to pass. Electric-typess will easily beat it too, but beware, as you might be hit by a Choice Banded Hidden Power Ground on the switch or Huntail can Rain Dance and get in a Surf / Hydro Pump. Even if you just attack it, it will go down with that crap HP, but anything not resistant to it—aside from Blissey, Snorlax, and Regice—doesn't take STAB Hydro Pumps in the rain very well.