Speaking on behalf of AU. (All clans had to have dominated for over 1 month, super clans which were made as a joke don't really count.)
#1 - Phoenix: Had the most stacked roster of all time, in any region. To be completely honest, we were pretty bad at everything. Just it's really hard to lose if every single one of your top lineup players can take a 1v4 and hardly ever dies. (Refer to Nick & Mitch 2v10)
#2 - Poets: A more disciplined version of Phoenix, player quality went down (Nick, Jimmy, Alex and Zane was still an amazing lineup). AU as a whole wasn't as good as it used to be when Poets was a thing so the PvP dominance was still there. (Refer to Nick and Jimmy 2v10)
#3 - Envy: This was literally a one man clan. Topgear was a crazy carry and single-handedly destroyed in team-fights. Only clan to take a legitimate round of Phoenix. (Won a 3v4 when they had less players. They still ended up losing 3-1, but that was a crazy play.)
#4 - Fade: Underrated due to their shitty reputation of hackers/DDoSing but like, went 70/3? In clan battles they were mostly legit, PvP was never as good as Phoenix or Poets but it was better than the majority of AU.
#5 - Apex: Really, really overrated by many people simply because they had a ton of wins. Despite their reputation of being the clan saints they really didn't play as fairly as people expected. They forged evidence to get Darkrai perm banned in the middle of Apex v Phoenix and threw a party afterwards, lol. Whenever I think of this clan I think of so much lost potential. They had really good players in Red_Dog, Musty, Blake and Flok but never used that lineup because the owners generally put themselves in. They played really defensively which hindered them imo. Despite this, they were still a really good clan.
EDIT: Honourable Mentions
#: NuggetZ: Probably better currently than 2013/14. Jed, Julian and Zane + 4th are a solid lineup.
#: Exile: Really let down by lack of depth in roster, Chris and Ryan as hard carries but without them it's a bunch of good players, full potential is great though.
#: Blitz: Above average in everything, no stand out strengths or weaknesses.
I really can't speak for EU clans and I only have a small opinion on US clans so I won't make a list, but globally AU Phoenix, US Forgotten and EU Forgotten were all the best at the way they played CBS. The thing is, US does 6v6-8v8, AU does 4v4-6v6 and EU does 2v2-4v4. All these clans played in different ways. US Forgotten would have decimated AU Phoenix in an 8v8, and AU Phoenix would have destroyed EU and US Forgotten in a 4v4.
Comparing these clans is impossible, I really want to say that Phoenix had the edge on PvP after playing in both AU and US but the players in EU Forgotten were pretty crazy too. (xExtract comes to mind, same with BomBashious for US)
Globally?
#1: AU Phoenix, US Forgotten and EU Forgotten.
#2 not better than the top 3 so doesn't matter
I hope that can give an insight to AU and compare the top 3 clans from each region in a global perspective, because each region is so different in it's community and different playstyles in CBS.
I'd love it if people could agree with my stance on how clans match up globally, if not I'm sure after some discussion we could come to a conclusion on the best global clan.