As of Gen 3, we had legendaries for every type barring Bug, Dark and Poison. And in Gen 3...those types were a bit eh, Dark arguably the least worse though, and exceptions naturally (Good ol Tyranitar, Houndoom, Heracross and Weezing).
Gen 4 bought a huge buff to Dark types with the Phys/Special split, Dark Pulse and a lot of new Darks (Drapion, Skuntank, Spiritomb) and evos for old, kinda sucky Darks (Honchkrow and Weavile). It also brought us the first Dark legendary Darkrai.
Gen 5 bought a huge buff to Bug types with their new almost-unique buff Quiver Dance and a lot of fantastic new members (Volcarona, Galvantula, Scolipede, really it's harder to name a Gen 5 Bug who sucks), and the first Bug legendary, Genesect.
Gen 6 may well be Poison's gen is what I'm saying. We've already got the rumour that Fairy is Poison weak, so Poison can now finally be 4x effective against something. Now we just need the awesome new Poison Pokemon, and the possible Poison legendary.
'Success rates are but scientific theory. A brave heart will defy any odds!' 'Don't beg for things, do it yourself or else you won't get anything.'