I was going to say Deoxys-S, because it was the pinnacle of HO while it was around, but Deoxys-D was also an extremely good hazard stacker for HO, albeit not nearly as successful, but still good. For this reason, I'm saving it for my #10 spot vote, and instead voting for another Pokemon that deserves to be on this list just as much as Terrakion:
Landorus-I
Shootinstarmie said most of what I'm going to say, but in my opinion, Landorus was pretty much a direct predecessor to Terrakion for being one of the most consistent Pokemon this entire generation. Although Terrakion was more of a factor because of its Speed tier and forcing the metagame to become more offensive, Landorus-I was well known in OU for being one of the hardest Pokemon to switch into. The key of Landorus' success lies in its extreme versatility that let it run a myriad of sets that all had separate counters, making Landorus one of the hardest Pokemon to prepare for in OU. Its physical sets dominated BW1, as its Choice Scarf set was an incredible revenge killer and hit extremely hard in the Sand, while its sweeping sets ranged from Swords Dance to even Mixed Expert Belt. It wasn't until early BW2 when people really got into using the deadly Life Orb Sheer Force sets that pushed Landorus over the edge, ultimately ending up in Ubers. This set had very specific counters depending on what moves it ran, but most of its common counters could be handled with the help of Tyranitar or even U-turn. Rock Polish Sheer Force was one of the most terrifying sweepers in OU that most teams had trouble dealing with, and it took a massive dump on offensive teams that gave it a chance to setup. Basically, this thing was extremely dominant and consistent Pokemon this generation, and it deserves this spot more than anything else I think.