Pokemon with event moves you can expect to see:
Gen 3 Events:

Wish: Chansey gained access to Wish through an obscure Generation 3 event, and ever since, has been a thing of legend, the type that everyone hates with every fiber being, unless it's on their side. Passing on monstrous 300+ HP Wishes is no mean feat, and it's something the fat blobs excel at. Fortunately, for those players who prefer using an actual DS, and game, don't expect to run into one, and if you do, odds are it has a bad nature, and/or mediocre IVs. Unfortunately, it's more common on simulators, so don't be surprised to run into this annoyance.

Wish: Much the same as Chansey above, Bagon was given out during the GBA era knowing Wish as well, and like Chansey, it's practically non-existent on retail carts, and the ones you do see more likely than not have terrible stats and natures. On simulators, it might be seen more often, but most players still prefer to take advantage of Salamence's bone-crushing Attack, rather then use a defensive set.
Generation 4:
Shadow Force + Spacial Rend: The supposed creator of all Pokemon got the three signature moves of its subordinates, however, just as with Dialga, Roar of Time is absolutely useless. On a brighter note, Shadow Force is the strongest Ghost-typed move in existence, hitting any Giratina trying to wall you hard, and the two-turn move can even be used to stall for Leftovers recovery (only on the normal forme of Arcues, though). On the flip side, your opponent can take advantage of that, and switch into something that doesn't care about eating a Shadow Force. Spacial Rend will never be used over Judgement on Arceus-Dragon, and most likely, won't be used on other formes either, although it's always something to keep an eye out for, since an unexpected Spacial Rend could be game changing.
Nasty Plot: Celebi certainly appreciated this addition to its movepool, making it even more unpredictable. Does your opponent have a support set, a slow boosting set, or an offensive set capable of smashing through your team? Celebi is more than capable of setting up Nasty Plot due to its resistances, and solid bulk, and it has an offensive movepool just large enough to take advantage of it.
Aura Sphere + Weather Ball + ExtremeSpeed + Zap Cannon: Raikou gained these moves at the expense of always being Rash, meaning that it can no longer tie with other base 115 Speed Pokemon (although it does allow it to reach 361 SpA). Fortunately, even with a Rash nature, Raikou still outspeeds max Speed base 100 Pokemon. Aura Sphere is a useful coverage move, allowing it to hit walls in UU like Snorlax for large amounts of damage, as well as having generally good coverage, and never missing. Weather Ball is an interesting option for those running Weather teams, however, it's generally better off using a more consistent move. ExtremeSpeed can be useful for picking off weakened opponents, but don't expect it to do much coming off of Raikou's relatively low Attack. Finally, it got Zap Cannon, which has the power of Thunder, and always induces paralysis, but comes with a measly 50% chance of actually hitting.
Flare Blitz + ExtremeSpeed + Howl: You can bet your ass that Entei will run these moves. Flare Blitz is the much appreciated physical Attack Entei had always longed for, and coming off of 115 base Attack, it hurts. ExtremeSpeed wasn't necessarily needed, but Entei certainly appreciates having a solid priority move, picking off weakened opponents, and compensating for the set Adamant nature of this event. Howl most likely won't be seen, but it's Entei's only way of boosting its Attack, outside of a Choice Band.
Sheer Cold + Aqua Ring + Air Slash + ExtremeSpeed: Suicune got the short end of the stick when the trio was released with special event moves, but that's not to say the moves it got are unusable. You will never see Sheer Cold while battling using standard Smogon tiers, but in VGC, it can take advantage of the absence of an OHKO clause, and use Sheer Cold in conjunction with a Resttalk set to pick off opponents. Aqua Ring can be useful for the additional healing, but Suicune already struggles to find an open slot for it, and would require it to drop either a coverage move, or a support move. Air Slash is unfortunately outclassed by Ice Beam, although it can be used to hit Ludicolo or Toxicroak hard, but otherwise has no merit. ExtremeSpeed is even less useful on Suicune than it is on Raikou, and will probably never be seen, unless your opponent was too lazy to teach it different moves. All in all, unless you really like Relaxed natured shiny Suicune, and are unable to come up with better one, you're better off using a non-event Suicune.
Generation 5:
V-create + Bolt Strike + Blue Flare + Glaciate: V-create in the sun on a Choice Banded Victini generally means at least one Pokemon is going to die. The strongest move in the game outside of Selfdestruct and Explosion, and it won't even miss because of Victini's ability. It does have the unfortunate side-effect of lowering its Defense, Sp. Defense, and Speed by one stage, but who the hell cares when you can slaughter just about everything that doesn't resist it, and tons of things that do? But that's only the tip of the iceburg; Victini also gets Zekrom's signature move, Bolt Strike. A 130 BP physical Electric move, and even better, Victini's ability again patches up the somewhat annoying accuracy of the move. Now, even Water-types can't switch in to sponge a possible V-create. Blue Flare won't be seen as often, due to V-create hitting much harder, but it's still something to prepare for. Like Bolt Strike, it has 130 BP, and like Bolt Strike, Victory Star patches up the accuracy issues. Glaciate allows Victini to hit Dragons hard, and has solid coverage with the Fire and Electric moves it normally carries, and as an added bonus, lowers your opponents Speed.