Can be a threat to standard teams if your opponent doesn't happen to have Celebi, as it hits about any standard wall quite hard.
Raichu is one of the fastest Encorers in the game. In standard, play it as follows: come in on a non-attacking move coming from a slower Pokémon and Encore it, then predict what your opponent is going to do. If Blissey, Snorlax or Regice are coming in, Focus Punch on the switch. If you think Swampert is coming, Hidden Power Grass it on the switch—it will not OHKO though. If you're insecure about prediction, use Substitute after Encoring, and handle from there. However, it's recommended to use Hidden Power Grass if your skills allow it, because Swampert is on nearly every team and it greatly hinders Raichu's sweeping potential.
For UU play, Focus Punch probably won't come handy, as Thunderbolt will 2HKO Aggron and Clefable is rare, and not much else weak to Fighting is around, besides Golem who you can OHKO with Hidden Power Grass. Therefore, the EVs suggested are for standard play. A guaranteed 2HKO on Blissey with Focus Punch, and enough Speed to outspeed all Heracross.
Use this set with Substitute and Hidden Power Grass in UU to get quite a threatening sweeper. Surf is the main reason to use it over Manectric, because it 2HKOs Gligar and OHKOs Camerupt. Only use Hidden Power Ice in standard for Flygon and Salamence (usually doesn't OHKO), because you need Hidden Power Grass in UU for Quagsire. Like with the first set, if you smell a standard Special wall coming, Focus Punch it.
When using Focus Punch, use the EVs suggested on the first set. Otherwise, use these. Timid Raichu ties with Jolly Fearow, Dodrio, Linoone and anything else with 100 base Speed and a +Speed nature. Modest ties with the same Pokémon, except with Adamant versions. You'll usually outspeed Salamence as well as the rare Charizard. Use the IVs suggested when using Hidden Power Ice in standard, having absolute max Speed is important.
Raichu can force switches easily, being an Electric-type. Predict this, Wish, and switch to whatever counters what they sent in. It can stay alive against Blissey by Wishing and then Substituting. Wish prevents it from using Surf, so you are stuck using Hidden Power.
OHKOs Blissey and other normal friends and is a nice surprise. They might think you are whittling down yourself to get a Petaya Boost, so take advantage of their surprise. Watch out for Venusaur and Dusclops. Unfortunately, Surf cannot be used in tandem with Reversal. Not really recommended in either UU or standard: in standard Tyranitar messes you up, and in UU there's a lot of Quick Attack and similar moves around.
Thief can grab Leftovers from Blissey and Snorlax. Charm can be annoying for Curselax and Swampert. Volt Tackle can give you some extra power, but it can't be used with Surf.
233 Attack with Focus Punch to 2HKO Blissey in standard. 299 makes you tie with most Salamence for Speed, and 328 makes you faster than stuff like Modest Gengar and the like. Put leftover EVs in Special Attack. When not using Focus Punch, just make sure to max Special Attack for maximal sweeping power.
Checks and Counters
In standard, Venusaur and Celebi can come in easily on Thunderbolt, Hidden Power Grass, Surf and Focus Punch, taking nil damage thanks to Leftovers and Recover, Leech Seed and / or Synthesis. Even Hidden Power Ice doesn't do that much. Dusclops with around 360 Special Defense can take quite a few Thunderbolts from Raichu if it can Pain Split or Rest in time, and is immune to Focus Punch, while Hidden Powers and Surf don't do much damage. Blissey, Snorlax and Regice must avoid Focus Punch at all costs, so they cannot come in effectively on a Substitute Raichu either, but the rest of its moves don't do much at all. Snorlax might even be able to come in on Substitute and Curse up and come out on top, but beware of Raichu using Encore.
Grounds can be used for countering Raichu to an extent. Flygon needs to beware of Hidden Power Ice, and doesn't take Focus Punches too lightly (especially if Flygon is Choice Banded and thus lacks Leftovers recovery), but makes an excellent counter nonetheless because of a guaranteed OHKO with its STAB Earthquake. Claydol is immune to Thunderbolt and resists Focus Punch, but is 3HKOed by Surf and almost 4HKOed by Hidden Power Grass or Ice. It is hard for Raichu to get around Claydol though, since Raichu will have to predict it switching in every time. Donphan is easier to beat but is still a hindrance; it takes nearly no damage from Focus Punch thanks to its high Defense, but Surf and Hidden Power will 2HKO him. Swampert is 2HKOed by Hidden Power Grass, but every other attack does absolutely nothing to it.
In underused, Camerupt can be almost completely disregarded as a counter because of Surf. Vileplume and Meganium, much like Venusaur and Celebi, take nearly no damage from Focus Punch and Thunderbolt and can shrug it off with Leech Seed and Synthesis. They should watch out a little more for Hidden Power Ice, though. Quagsire and Whiscash do the same job as Swampert: countering any variant not using Hidden Power Grass with no trouble at all. That's why Vileplume or Meganium combined with Quagsire or Whiscash walls any Raichu available. Sandslash and Gligar do the same as Donphan here: being 2HKOed by Surf (and in Sandslash' case, Hidden Power) doesn't cut it, but easily take Focus Punch and Thunderbolt for breakfast. Hypno and Grumpig, the standard special walls of UU, do quite nicely at taking anything Raichu throws at them, though Grumpig has no Wish like Hypno does, so it will get worn down quickly. A sweeping Grumpig doesn't like to get Thunder Waved either.
In general, attacking Raichu straight on is a good idea if it doesn't OHKO you. It's very fragile and weak to the most common attacking move in the game (Earthquake), and it often uses Substitute to see what you're going to do. Just switching out if it comes in on your non-attacking move is handy as well, since it is probably going to use Encore.