Everything that dies to Stealth Rocks, 'cause you know, if you play standard there'll be those damn rocks EVERYWHERE.
Basically i don't play stuff, that is weak against alot of the current used attacks.
Sure you'll balance your team out to have switch-ins to soak up the damage, but still.
I'd say flying und bug types probably. Because outside of Heracross most bug's aren't worth it in OU and fliers.. meh. Low stats, mediocre attacks and a often horrible combos and flyin' alone just save you from EQ.
@Darkryter
Poision is not that unused. Gegnar and Tentcruel are poison types and they're OU pokemons.
And if we're talkin out side of PvP just the normal game: I use my starter.
That's all you need to clear every pokemon game i've ever played
Play with the starter, catch the Plot-Legendary + one legendary with your masterball and rofl-stomp the Top4 with 3 pokemons ~10-20lvls higher then theirs.
If you want to overkill the NPCs backed them up with a Chansey/Blissey. Though Chansey + Evolit means you're basically immune to the S.Atks of the NPCs^^
PS: I just read Azaradel's post
Of course Ice. It's probably the worst type out of all types.
Fighting + Fire are probably one of the most used attacks (To fight Ferrothorn e.g) and takes a supereffective from stealth rocks.
That's why Arcticuno, even though he's a legendary bird, fights in the Underused bracket. Takin' 50% Hp dmg on entrance is just lulz.