Houndoom

Type Tier
Dark / Fire BL
Statistics
Stat
HP
75
353
Atk
90
278
Def
50
198
SpA
110
318
SpD
80
258
Spe
95
288
Name Item

Pursuit

Leftovers
Moveset
~ Crunch
~ Fire Blast
~ Pursuit
~ Counter

Use Fire Blast or Crunch depending on what you think is going to come in. Houndoom can survive all non-STAB Earthquakes and all STAB Returns, so use Counter when they do that for a quick KO. And if something's on the brink of fainting, but might switch out, use Pursuit to take care of it.

Name Item

Sleep Talk

Leftovers
Moveset
~ Fire Blast
~ Crunch
~ Rest
~ Sleep Talk

Sleep Talk gives back some Fire Blast PP. Exeggutor isn't going to get by this one unless it Explodes.

Name Item

SunnyBeamer

Leftovers
Moveset
~ Fire Blast
~ Crunch
~ Sunny Day
~ SolarBeam

This is an old school set. Take half damage from Water attacks while you SolarBeam them. This set is completely ruined by Blissey and Snorlax, but is very dangerous in underused play since there aren't a lot of things that can survive all three attacks.

Other Options

If you want to try it out, Endure + Reversal can catch a lot of things off guard. While Houndoom's Reversal isn't very strong, it will mess up Blissey (OHKO), Snorlax (73% damage), Umbreon (70%), Tyranitar (OHKO), and Rhydon (63%). Watch out for Spikes, however. If you're really worried about Rhydon, there's always Hidden Power Grass / Hidden Power Water. Another interesting option to note is Beat Up. If everything on your team is in perfect shape, you've more than likely got a 2HKO on Blissey. However Blissey is the ONLY thing that Beat Up will deal significant damage to.

Opinion

Houndoom is arguably the best Fire-type in the game (though most would probably swear by Charizard). Houndoom is the only Fire-type that can effectively mess up three of the main Water-types (Cloyster, Slowbro, and Starmie), since none of them want to switch into a STAB Crunch from a 318 Special Attack. It's faster than most commonly-used things in GSC, and Pursuit royally messes up stuff that tries to switch out. While Houndoom's type combination doesn't really help or harm it one way or the other in terms of being resistant to certain attacks, Dark and Fire attacks work pretty well together and are super-effective on a lot of popular types.

Counters

Suicune and Tentacruel's high Special Defense and Water typing resist Fire and ruin Houndoom with Surf. Quagsire and Kingdra work similarly, to a lesser extent. Blissey is a basic special wall to sponge Fire Blasts with, but don't expect it to get past the Sleep Talk set. Umbreon can't do any damage, but Mean Looks and Baton Passes it off. Snorlax, Tyranitar (watch out for the odd Reversal), Muk, and Dragonite take all of Houndoom's attacks quite well and respond by hitting its very weak Defense. Rhydon and Golem resist Fire and OHKO Houndoom, but their low Special Defense won't let them take too many Crunches.