In my own view:
Wolf Empire Pro-Argument: All the plot armor of House Davion AND the Harlem Globetrotters combined. Their leader is named after Alaric, they were able to prosecute not one but two wars of conquest without supply or lower castes, they have the Second Coming of Natasha Kerensky, and of course there's all those meta trajectory tidbits that all point to a Wolfen ilClandom.
Wolf Empire Con-Argument: If one were to not acknowledge that the Wolves are the Gary Stus of the BTU, here's where they might be tripped up. As the Wolf Empire, the Wolves are now a warrior caste without lower castes. They're not even a complete Warrior Caste, as they've had to enroll indig planetary miltia into the hallowed ranks of their Touman. As 2nd line warriors, but warriors no less. If their plot armor fails them on their leap to Terra, they've got absolutely no safety net at all. They're going to be swallowed by Sphereoids and the Empire will just become another Successor State without all the non-warrior trappings of Clan culture.
Ghost Bears: Self-eliminated from contention as strongest Clan, as they've gone Native. However, if one were to continue to consider a branch of the Rasalhague Dominion as still being Clan...
Pro-Strongest Clan Argument: They've dispensed with much of the inherent silliness that hamstrings Clan operations. They fight wars intelligently now, and for some reason have the resources that Kurita never did when it controlled the Rasalhague Military District.
Con-Strongest Clan Argument: Well the whole "they're not Clan anymore" thing. Besides that, strategically they just sit and sandbag. In the latter half of the 31st and first half of the 32nd century just sitting there and letting fate deal you a lucky hand worked out for them, but it'd be sloppy storytelling for the ilClandom to just fall into their laps without their having sought it out.
Jade Falcons:
Pro Argument: Malvina has taken what it means to be Clan to the logical extreme. If anyone deserves to be ilClan, it's the Falcons. Ruthless, fearless, and not at all afraid to break a few billion eggs to make an omelet.
Con Argument: They're not the Wolves. No way will they have anything nice if it comes at the expense of the Wolves. Right now they're positioned in the same place in the story as House Liao was right on the eve of the 4th SW.
Sea Foxes/Clan Protectorate: Honestly, if the events of the BTU were not predetermined, I'd pin the Sea Foxes as the strongest Clan. There's a vacuum left by ComStar, and the Sea Foxes are the only ones who can fill it. And they certainly should.
Sea Foxes Con argument: Arguably, they're no more true "Clanners" anymore than what used to be the Ghost Bears. Even if they do take over as the sinister kingmakers and power brokers of the Inner Sphere, it might not count as being the most powerful Clan.
Raven Alliance Pro-Argument: They got the warships.
Raven Alliance Con-Argument(s): This is a game of stompy mech action. Having the best WarShip fleet is about as good as a football team having the best cheerleaders. Flashy and draws fans, but doesn't win where it counts. Besides, they're chronically minor league. Went from being a Homeworld Clan to being half of a minor Periphery faction. Going sideways rather than upwards on the scale there.
Hells Horses: I got no pro-argument for them. Their role has been, at best, Malvina's Mooks. Now they're not even that. Unless the mysterious Operation NOYAN involves ambitions and/or heretofore unknown faction(s) entering the Game of Space Thrones, they're just disenfrachised small fry.