Okay, so I think we need to have a discussion about this because it affects a few of the recommended sets.
In GSC, Roar and Whirlwind fail if they go first, to prevent Sleep Talk Roar shenanigans. (In ADV, they allowed Roar and Whirlwind to work if they went first.) This means that if two Pokemon both use a p-haze move, the slower one wins.
In turn, this means that the optimal IVs and EVs for Speed are not necessarily 15/63002. Pokemon using p-haze moves have to weigh up whether they want to go first or second, and even against which targets they want to go first or second.
(Achieving 0 Speed EVs is possible, by the way. Fighting Shuckle with an Exp.Share and 3 Pokemon in the party will give zero Speed EVs and nonzero of everything else; for RBY Pokemon and their evolutions there's also Slowpoke, although it always gives at least 1 Speed EV in GSC.)
Some of the more interesting Pokemon for Speed reduction:
Raikou
The only things speedtied with Raikou are Starmie (quite uncommon, and still loses unless Raikou is on a sliver of health) and other Raikou (found on almost every stall). Raikou dittos are quite common in a stall vs. stall matchup, as Raikou is one of the best switches into itself, and the "winner" of this is a Raikou that can Roar out the opposing Raikou and start a Spikes shuffle. As such, Roar Raikou generally wants to drop its Speed so it can win Roar wars with other Roar Raikou (Raikou without Roar obviously still wants max Speed, as it can't win Roar wars anyway).
Notable stopping points for Raikou's Speed drop are:
319: Still beats Gengar and Espeon. Raikou is usually the only Pokemon on a stall team which outspeeds Gengar, so rare Destiny Bond Gengar can be difficult to kill if you go below this and Reflect/Roar Raikou won't be able to put up Reflect in time to block its Explosion. Espeon is rare, but insofar as their matchup usually comes down to a slugging match Raikou generally wants to go first against it. In Ubers there is also Lugia to consider, but it nearly always switches out (and may have Whirlwind of its own) so outspeeding it's not especially desirable.
299: Still beats Zapdos, Miltank, Charizard and Tentacruel. Raikou generally doesn't want to go lower than this, because the first two matchups are very common (and Raikou wants to go first in both) and while the latter two are much rarer Raikou is usually one of the main things preventing a team from getting swept by them. In Ubers there is also Celebi, which is one of Raikou's main walls, but unless Raikou is running Hidden Power Bug this doesn't matter very much.
269: Still beats Nidoking, Suicune and Misdreavus. Raikou usually wants to switch out of Nidoking, but if Raikou is HP Ice/Water and Nidoking switched into Hidden Power, Raikou wants to go first so it can secure the KO. Suicune is almost always going to switch out of Raikou unless it's Mirror Coat (in which case being faster won't really help). Outspeeding Misdreavus doesn't matter all that much, as its only set that threatens to KO Raikou is Perish Trap and Speed is nearly irrelevant there.
237: Minimum Speed Raikou without going to 0 IVs and losing HP.
235: Minimum Speed Raikou.
Jolteon
Jolteon shouldn't bother with dropping Speed if it's running Agility, as it'll outspeed everything anyway. If it's running Growth, though, Roar Raikou (one of GrowthJolt's main walls) is potentially vulnerable to a slower Roar from Jolteon (though this requires Jolteon to either give up Rest or Hidden Power). Obvious places to stop are the same as Raikou's, except Jolteon's minimum Speed values are 267/265.
Lugia and some Lugia counters
(Obviously, we're talking about Ubers here.)
Whirlwind Lugia (Aeroblast/Curse/Recover/Whirlwind) is pretty common in Ubers and any good team carries at least one answer to it. One option is obviously the legendary Electrics, but another is slow Flying-resist phazers (Skarmory, Tyranitar, Rhydon and Steelix) that are slower than -1 Lugia. Lugia can potentially get around these by lowering its Speed, so it's common to counter-slow these Pokemon if they're to be relied on in this role (Skarmory and Tyranitar need their Speed lowered to work at all). In practice, Lugia will often keep its full Speed, as -2 Lugia's one-point Speed advantage over Snorlax can be relevant against Belly Drum sets, but assuming you'll never run into a Speed-dropped Lugia is foolish.
Others
There's CurseSubRoar Kangaskhan vs. Toxic Skarmory, but that's a relatively obscure matchup and Kangaskhan outslowing Skarmory will by necessity also be outsped by Snorlax after a Curse. Skarmory vs. Skarmory is a potential driver of Speed anti-creep, but this matchup is almost never sought out so it's very rare and can generally be ignored. Steelix can give up the speedtie with Snorlax to outslow -1 Skarmory and use it as Roar or setup bait (if it does this, it should go all the way to 1 IV/0 EVs).
In GSC, Roar and Whirlwind fail if they go first, to prevent Sleep Talk Roar shenanigans. (In ADV, they allowed Roar and Whirlwind to work if they went first.) This means that if two Pokemon both use a p-haze move, the slower one wins.
In turn, this means that the optimal IVs and EVs for Speed are not necessarily 15/63002. Pokemon using p-haze moves have to weigh up whether they want to go first or second, and even against which targets they want to go first or second.
(Achieving 0 Speed EVs is possible, by the way. Fighting Shuckle with an Exp.Share and 3 Pokemon in the party will give zero Speed EVs and nonzero of everything else; for RBY Pokemon and their evolutions there's also Slowpoke, although it always gives at least 1 Speed EV in GSC.)
Some of the more interesting Pokemon for Speed reduction:
Raikou
The only things speedtied with Raikou are Starmie (quite uncommon, and still loses unless Raikou is on a sliver of health) and other Raikou (found on almost every stall). Raikou dittos are quite common in a stall vs. stall matchup, as Raikou is one of the best switches into itself, and the "winner" of this is a Raikou that can Roar out the opposing Raikou and start a Spikes shuffle. As such, Roar Raikou generally wants to drop its Speed so it can win Roar wars with other Roar Raikou (Raikou without Roar obviously still wants max Speed, as it can't win Roar wars anyway).
Notable stopping points for Raikou's Speed drop are:
319: Still beats Gengar and Espeon. Raikou is usually the only Pokemon on a stall team which outspeeds Gengar, so rare Destiny Bond Gengar can be difficult to kill if you go below this and Reflect/Roar Raikou won't be able to put up Reflect in time to block its Explosion. Espeon is rare, but insofar as their matchup usually comes down to a slugging match Raikou generally wants to go first against it. In Ubers there is also Lugia to consider, but it nearly always switches out (and may have Whirlwind of its own) so outspeeding it's not especially desirable.
299: Still beats Zapdos, Miltank, Charizard and Tentacruel. Raikou generally doesn't want to go lower than this, because the first two matchups are very common (and Raikou wants to go first in both) and while the latter two are much rarer Raikou is usually one of the main things preventing a team from getting swept by them. In Ubers there is also Celebi, which is one of Raikou's main walls, but unless Raikou is running Hidden Power Bug this doesn't matter very much.
269: Still beats Nidoking, Suicune and Misdreavus. Raikou usually wants to switch out of Nidoking, but if Raikou is HP Ice/Water and Nidoking switched into Hidden Power, Raikou wants to go first so it can secure the KO. Suicune is almost always going to switch out of Raikou unless it's Mirror Coat (in which case being faster won't really help). Outspeeding Misdreavus doesn't matter all that much, as its only set that threatens to KO Raikou is Perish Trap and Speed is nearly irrelevant there.
237: Minimum Speed Raikou without going to 0 IVs and losing HP.
235: Minimum Speed Raikou.
Jolteon
Jolteon shouldn't bother with dropping Speed if it's running Agility, as it'll outspeed everything anyway. If it's running Growth, though, Roar Raikou (one of GrowthJolt's main walls) is potentially vulnerable to a slower Roar from Jolteon (though this requires Jolteon to either give up Rest or Hidden Power). Obvious places to stop are the same as Raikou's, except Jolteon's minimum Speed values are 267/265.
Lugia and some Lugia counters
(Obviously, we're talking about Ubers here.)
Whirlwind Lugia (Aeroblast/Curse/Recover/Whirlwind) is pretty common in Ubers and any good team carries at least one answer to it. One option is obviously the legendary Electrics, but another is slow Flying-resist phazers (Skarmory, Tyranitar, Rhydon and Steelix) that are slower than -1 Lugia. Lugia can potentially get around these by lowering its Speed, so it's common to counter-slow these Pokemon if they're to be relied on in this role (Skarmory and Tyranitar need their Speed lowered to work at all). In practice, Lugia will often keep its full Speed, as -2 Lugia's one-point Speed advantage over Snorlax can be relevant against Belly Drum sets, but assuming you'll never run into a Speed-dropped Lugia is foolish.
Others
There's CurseSubRoar Kangaskhan vs. Toxic Skarmory, but that's a relatively obscure matchup and Kangaskhan outslowing Skarmory will by necessity also be outsped by Snorlax after a Curse. Skarmory vs. Skarmory is a potential driver of Speed anti-creep, but this matchup is almost never sought out so it's very rare and can generally be ignored. Steelix can give up the speedtie with Snorlax to outslow -1 Skarmory and use it as Roar or setup bait (if it does this, it should go all the way to 1 IV/0 EVs).