Tyranitar still holds the record at 7 consecutive generations OU. However, if modern tiering policy existed in Gens 3 and 4, it would have been banned back then; it also dropped to UU for a month in early SS before returning.
Of all the Pokemon whose place in OU has never changed or been questioned, Heatran holds the record at 6 consectutive generations. Landorus-T is the runner-up at 5 consecutive generations.
There are a few other edge cases, though:
-Skarmory ties Heatran at 6 generations OU, but dropped to UU in SS and was then cut from the Pokedex.
-Magnezone ties Landorus at 5 consecutive generations OU, but has dropped to UU in SV.
-Gliscor would very obviously tie Heatran at 6 generations, but has been cut from the Pokedex.
-Ferrothorn would very obviously tie Landorus at 5 generations, but has been cut from the Pokedex.
-Zapdos has been OU for 8 of the 9 generations, but these aren't consecutive and its drop to UU was right in the middle at BW.
-Chansey OR Blissey has been OU for 8 consecutive generations, but because they strictly compete with each other neither has held more than 3 consecutive generations individually.
