For example. Take a look and compare the different Smogon OU tierlists over the past seven generations. Aside from a few mainstays like Gengar or Tyranitar, newly released pokemon games can drastically change the newest OU tierlist up from the previous one.
>Smogon's ou tierlist for (Gen 1)
http://www.smogon.com/dex/rb/formats/ou/
First gen; no returning ou pokemon, because there is no previous gen.
>Smogon's ou tierlist for (Gen 2)
http://www.smogon.com/dex/gs/formats/ou/
Returning pokemon from gen 1: Cloyster, Exeggutor, Gengar, Jolteon, Rhydon, Snorlax, Starmie, Zapdos
>Smogon's ou tierlist for (Gen 3)
http://www.smogon.com/dex/rs/formats/ou/
Returning from gen 2: Blissey, Cloyster, Gengar, Heracross, Jolteon, Porygon2, Raikou, Skarmory, Snorlax, Starmie, Suicune, Tyranitar, Zapdos
>Smogon's ou tierlist for (Gen 4)
http://www.smogon.com/dex/dp/formats/ou/
Returning from gen 3: Aerodactyl, Blissey, Celebi, Flygon, Forretress, Gengar, Gyarados, Jirachi, Jolteon, Metagross, Skarmory, Snorlax, Starmie, Suicune, Swampert, Tyranitar, Zapdos
>Smogon's ou tierlist for (Gen 5)
http://www.smogon.com/dex/bw/formats/ou/
Returning from gen 4: Blissey, Breloom, Celebi, Dragonite, Forretress, Gengar, Gliscor, Gyarados, Heatran, Hippowdon, Infernape, Jirachi, Jolteon, Lucario, Magnezone, Mamoswine, Metagross, Rotom-Wash, Scizor, Skarmory, Starmie, Tentacruel, Tyranitar, Vaporeon
>Smogon's ou tierlist for (Gen 6)
http://www.smogon.com/dex/xy/formats/ou/
Returning from gen 5: Breloom, Dragonite, Dugtrio, Excadrill, Ferrothorn, Garchomp, Gengar, Gliscor, Heatran, Hippowdon, Jirachi, Keldeo, Kyurem-Black, Landorus-Therian, Latias, Latios, Magnezone, Rotom-Wash, Scizor, Skarmory, Starmie, Tyranitar
>Smogon's ou tierlist for (Gen 7)
http://www.smogon.com/dex/sm/formats/ou/
Returning from gen 6: Bisharp, Chansey, Charizard (both megas), Clefable, Diancie-Mega, Dragonite, Excadrill, Ferrothorn, Garchomp (both forms), Gengar, Heatran, Keldeo, Kyurem-Black, Landorus-Therian, Latios (both forms), Lopunny-Mega, Magnezone, Manaphy, Medicham-Mega, Mew, Pinsir-Mega, Scizor-Mega, Skarmory, Tangrowth, Tyranitar (both forms), Venusaur-Mega, Zapdos