Starmie for 5 because of very strong RBY and DPP presences and its integral role as a spinner in the GSC and ADV metagames. Its existence is the main thing motivating Chansey usage in RBY (even though Lapras is what should be scaring people into using Chansey), and in DPP it's just plain fantastic as a LO sweeper or a defensive spinner or even as a stallbreaker thanks to Trick.
This is followed closely by Skarm for 6. It's been the premier physical wall in every metagame it's graced. I think the main thing that keeps me from putting it over Starmie is that in GSC what made it so good wasn't so much its own attributes but rather how fantastic Snorlax was. So while it's arguably a top 5 mon in GSC, that's really deceiving because it's merely reacting to the most dominant Pokemon in any OU metagame rather than setting the tone itself. Also not being in RBY hurts it too.
After that Suicune works, there's nothing more deserving imo. I might be short-selling Suicune here, I mean it's always been a relevant threat since its introduction, but it's never really defined any single aspect of its metagame as much as Starmie or Skarm has. While it's always been a defensive juggernaut, there's always been other things that could wall what it could, granted not with the same breadth of defensive coverage but usually with better defensive characteristics against the threats that mattered more. Also, while it CMs in later gens, I get the impression that these CM cunes are much better on paper than in practice. Much like Tobybro.
Also for #8 Blissey should be considered. Granted it was Chansey in RBY so it doesn't count and in GSC it sucked balls, but it makes a very strong ADV showing and of course is the definitive special wall of DPP.