>>37249457Steel is self-explanatory, best defensive typing in the game and one of the few checks to Fairy.
Fairy is god tier both offensively and defensively. Pairs well with Steels, forms a good STAB combo with almost any dual-type, synergizes very well with common coverage moves like Fire/Fighting/Ground. Not many good stops to Fairies, and running Poison or Steel coverage is awkward and usually done solely for them.
Water for similar reasons as Fairy. Scald exists, Ice Beam exists, Rain exists. Half of the reason fat Grasses exist are to stop Waters (the other half being EdgeQuake).
Flying is more of a recent thing, but it's always been good offensively as far as the type chart. Before Gen IV your best Flying move was 70 BP Hidden Power Flying (or Drill Peck, but that doesn't help Gyarados or Salamence). Now that you have Brave Bird, Hurricane, Pinsir, and Z-moves, you can actually make use of its lack of resists. Defog and Fighting/Ground resists don't hurt either.