Insomnia Delibird
TROLL MOAR GAMEFREAK!
(If you're wondering about that comment, it is because Delibird already had VITAL SPIRIT as a natural ability)
Anyways, I've found a lot of good Dream World abilities, some who were even underlooked.
Imposter: This changed Ditto into the Edward VII of Pokemon into one of the scariest Pokemon to ever encounter. + 6 Salamence raging against your team? Send that Ditto out to revenge that monster... slap a Scarf on it, click Outrage, GG.
Shadow Tag: This broke Chandelure and made her one of the game's best revenge killers. Just slap a Scarf around and you basically choose who will go or who will stay... Gothitelle can also use this to some extent quite well.
Speed Boost: No need to explain here. So they paid attention to Infernape outclassing Blaziken, and said, "We can't have this!" and decided to 'balance it out'. Unfortunately, this decision was an overkill and pretty much broke Blaziken. Aside from the fact Blaziken was lacking speed, it was also lacking a reliable base 120 Fighting move. It had Superpower, but that meant it was going to come in and switch out due to the attack drops. Now it got a 130 BP Hi Jump Kick, which can basically OHKO most bulky waters after a + 2. Thankfully, that move has a huge drawback.
Multi-Scale: Only two Pokemon gets this, and while this makes Lugia even tougher to KO, Pressure might be better overall for PP stalling. Talking about Dragonite here... it has a CHANCE to survive a Specs Latios's DRACO METEOR with this ability. Multi-Scale made it much easier to set up Dragon Dances and wreak havoc thanks to its bulkiness and access to Roost and powerful Outrage. The drawback is it needs to be at full health to activate, but still, it is better than nothing and it helps Dragonite set up quite readily.
Harvest: Often overlooked but still quite a scary thought if you realise this means INFINITE berries. Sitrus Berry + Sub + Swords Dance + Wood Hammer Exeggutor is quite a force to be reckoned with, and Lum Berry means immunity to status. Once Starf Berry comes out, prepare for a makeshift "Moody" without the - 1 drawback. Tropius is also pretty cool with this ability, especially in the sun and access to Growth.
Prankster: Turned Sableye from Zero to Hero. We know exactly how annoying Whismiscott is, and + 1 priority to passive moves is certainly no slouch. It gets annoying and allows stalling without the use of speed EVs. Of course, people do go overkill with Whimsicott's speed, but is all that speed necessary to outpace Thundurus or should we buff more into HP/Def to survive Ferrothorn's Gyro Balls? Up to the trainer's of course.
Magic Bounce: Congrats Espeon and Xatu. You are now used more often than you were from before thanks to being able to reflect back pretty much a lot of passive set up moves like Stealth Rocks or Thunder Waves. Well, more Espeon than Xatu, but still. This is one scary ability.
Contrary: One of the scariest abilities if it were given to, I don't know, Heracross or something. Regardless, Serperior was given this ability, and like the tricky serpent from the Garden of Eden, everything it does is a lie. Leaf Storm gives + 2 SpA as opposed to reducing it, making her one scary thing. However, Serperior does not really have any other coverage move to go alongside it, aside from some random Hidden Power, which is disappointing. However, it does balance her out otherwise this ability would be broken off stuff that like to lower their own stats with Superpower or the scary Draco Meteor.
Imagines Contrary V-Generate Victini and runs
Drizzle: This was what made Kyogre scary. Now give this to a bulky froggy with a plethora of moves and with infinite rain, you can just throw a cheap rain team in for some swift swim sweep. And Politoed is not even dead weight... it can seriously mess stuff up. It turned out so well that Swift Swim + Drizzle is banned together.
Drought: Same deal with Drizzle, but of less influence. Regardless, this makes Ninetales quite scary, as she can revenge Excadrills who like to play in the sand for a long time and provide support for Chlorophyll Pokemon like Venusaur.
Chlorophyll: What happens when you give something already versatile and powerful and bulky with double speed? Oh hai Venusaur... with Chlorophyll and access to Growth in the sun this thing becomes a mean sweeper. Sleep Powder incapacitates counters and it has a choice of STAB Solarbeam/Sludge Bomb and Hidden Power [Fire]/Earthquake to work with. Only problem is he does not have room for all of them!
Cloud Nine: If this generation became weather happy then this was definitely needed. Pokemon like Golduck is good, but usually overlooked for other waters because it is too one dimensional, so why not give it to something like Lickilicky? After all, it is a bulky normal with a widespread movepool, enough to deal with a lot of weather (ab)using Pokemon.
There is a LOT to list, but I won't go there for now.