I'm going to answer this, as the existing answer is somewhat incorrect, both in Pokemon and in reasoning.
Wobbuffet was UUBL in BW, then Untiered in XY.
Politoed was OU in BW, then Untiered in XY.
Leafeon was UU in DP, then Untiered in BW.
Dugtrio was OU in XY, then Untiered in SM.
Gothitelle was UUBL in BW, then Untiered in XY.
Charizard was UUBL in RS, then NU in DP.
Wobbuffet: In BW, it was UUBL because of the Encore nerf. It faced competition from Gothitelle, a more offensive Shadow Tag user. Despite this, it managed to hold its own in OU. However, in XY, Shadow Tag was banned in all tiers except Ubers. While it was somewhat viable in Ubers, it was heavily crippled in all other tiers, thus shooting it and Gothitelle down to Untiered. Shadow Tag was later banned in Gen 5 OU as well, but that happened after the Gen VI metagame began.
Politoed: Weather teams became less common in XY with the weather nerf, meaning Politoed was used on fewer teams. Drizzle was also banned in all tiers except OU, and so Politoed struggled to find a niche anywhere, and as such went to Untiered.
Leafeon: In BW, Sawsbuck was introduced. With two immunities with Sap Sipper, the same Speed and a slightly lower Attack stat than Leafeon, and Horn Leech, Double-Edge, Jump Kick, Substitute, and more, Leafeon was almost completely outclassed. It was still viable in lower tiers, but Sawsbuck could do almost everything Leafeon could, but better.
Dugtrio: Dugtrio should not have been OU in XY after the ban of Arena Trap, but it was. This was immediately remedied in SM, where it went straight to Untiered.
Gothitelle: In BW, it was UUBL because of Shadow Tag, allowing it to easily trap and KO specific opponents. In XY, Shadow Tag was banned in most tiers, and without it, Gothitelle couldn't find a niche, and fell to Untiered. Shadow Tag was later banned in Gen 5 OU as well, but that happened after the Gen VI metagame began.
Charizard: Two words: Stealth Rock. Charizard took 50% damage every time it switched into a field with Stealth Rock up, making it very easy to KO. It could still be useful with Rapid Spin support, but Stealth Rock greatly crippled it. It was better in RS because there was no Stealth Rock.
I'll keep researching, and if I find any drops more drastic than the ones on this list, I'll edit it.
Hope I helped!