For #8 - #10 I repost all the options
Blissey is not very good in GSC, but it has a great impact in ADV and DP.
Celebi is top 3 in ADV, and very good in DP
Vaporeon is a big threat in GSC, and a very good bulky water in ADV/DP, giving Wish support too, but it sucks in RBY.
Jolteon is decent in RBY, a good agility passer in GSC, and decent in ADV and DP.
Dragonite is excellent in RBY (wrap), a good mixed sweeper in GSC, its decent in ADV and a monster in DP (especially after mence ban). It could easily be at least top 10.
Heracross is decent in GSC and DP, and a huge threat in ADV.
Forretress is a good option for a spiker + spinner in GSC; in ADV although it not a good spinner (unless it has pursuit support), it can spike and spin, and it has useful resistances; in DP it can set up all the hazards, and it can also spin.
Cloyster is a great option in RBY vs snorlax/tauros and can clamp and explode; in GSC it's the best spiker, and has a good explosion (usually removing things like Starm/Lax), and can also spin; in ADV it is the only spiker in OU/BL that isnt trapped by Magne. It sucks in DP though.
Machamp is not good in RBY, but its a huge threat in GSC and DP, and it can also be useful in ADV.
Raikou is top 3 in GSC, a huge threat in ADV (it fears Duggy though), and it can also be dangerous in DP.
Salamence didnt exist in GSC and RBY, but it was probably top 5/6 in ADV, and it was considered broken in DP. Maybe not top 10 because being present in 2 gens (but if we consider Celebi as Uber in GSC, Celebi is then in a similar case).
What do you think about machamp? It could be #10
I dont like Forretress in any gen. In ADV and DP as a Spiker its worse than Skarm, and as a Spinner it p bad, because it will never beat Gengar (GSC/ADV), and it will have a very hard time with rotom-a (DP).
Im not sure about Celebi, it was OU in only 2 gens