Ugh this is getting really difficult. At the end of the day the things competing for 8-10 all kinda blur together in terms of "dominance" and some honorable mentions might be in order.
Exeggutor is interesting. Mandatory in RBY, and definitely a top threat in GSC. The problem is, a Pokemon being garbage in 2 gens after its dominance is worse than one not existing for 2 gens prior to its dominance. At least in the latter case you can give that Pokemon the benefit of the doubt. I know I prefer mons on this list to actually stand out as a top-top threat in a metagame, let alone two, but at the end of the day, a certain degree of consistency is required for a Pokemon to be on this list, something Egg just doesn't have. I vote nay to Egg.
Dragonite has no place here. It's mediocre in GSC and ADV, it's definitely an off-beat option in RBY that wasn't even recognized as a threat for most of its history, and in DPP its greatness waxed and waned with bans and the distribution of Outrage. It's never really had footing as a dominant force until very late DPP. Nay for Dnite.
As for Cloyster, I'll agree with Hip. Cloyster is only a "solid" OU in GSC, even there it's more that Spikes are OU rather than Cloyster itself (and Cloyster isn't necessarily even the best Spikes-layer). Everywhere else it's second-class OU at best. Nay for Cloyster.
Celebi I'll have to reject too. I was under the impression that we weren't going to count Celebi in GSC, because if we did it'd be a lot higher than #8. If you discount GSC, it's top 3 in ADV and mid-tier OU in DPP. It's a lot like Jirachi who's mid-tier OU in ADV and top-tier OU in DPP. Neither, I think, has a strong enough showing in merely 2 gens to be considered for this list (I'd agree with Borat that you'd need to really dominate those two gens to make your way onto this list). Nay to Celebi. And nay to Jirachi, too.
Not a big Raikou fan myself. It's top 3 in GSC and definitely a big threat in ADV, but in ADV it's closer to mid-tier OU than high-tier and in DPP it's not OU at all. Nay to Raikou.
So, what do I like for this list? Well,
Blissey - The claims of it being garbage in GSC are exaggerated. I mean, let's get this clear, it's not that great, but it's definitely mid-to-low tier OU in GSC and has a lot of historical usage. Plus it's one of two Heal Bellers and, unlike Miltank, the things Blissey walls are walled forever, regardless of status or Spikes or crits or anything. It definitely shapes the nature of offense in GSC. As far as ADV and DPP are concerned, it's a highly-used, extremely dominant Special Wall. Nothing Special sponges like Blissey, and its dominance in this niche has made it extremely relevant in every generation it's appeared.
Forretress - I said it before, but here's a quick recap: the ultimate Spiker. Granted, there's always been competition, but Forretress' combination of Spikes, Spin, and typing have always made it arguably the best hazard setter available in every OU metagame it's appeared. Its dominance in this niche can't be denied, even if it is ultimately a role player.
I know I said I'd go with Forr at #8, but I'd probably go with Bliss at #8 and Forr at #9 now that I think about it.
Looking back on it, that's only 2 things I really like, and they're more niche Pokemon than the other stuff on this list. But I feel like if a Pokemon dominates a niche so resoundingly, even though it's not dominating every phase of the game the same way Snorlax or Tyranitar is, I'd say it deserves to be put on this list before an Exeggutor who had the every-phase domination thing for 2 gens but then fell off the face of the earth, or a Raikou who really only had the every-phase domination thing going for it for one gen.
As for #10, well, I'm still trying to decide what I might like, so consider my "nay" votes above as only pertaining to inclusion among the top 9. Except for Dnite and Cloy, they honestly have no business here.