If I were to choose for Most Powerful (at the start of the game) it would be between the Silver-bloods, the Black-Briars and the Snow-Shods. Silver-Bloods due to their control of Markarth, the Reach and (Most of) the Forsworn; Black-Briars due to their control of Riften, the Mead-Industry and the Thieves Guild; Snow-Shods due to their link to the Black-Briars and, via Vittoria Vici, the Emperor. I'd choose the Silver-bloods since they have the Forsworn at their disposal and the greatest deposit of silver in all of Skyrim for free whilst the Black-Briars have a Mead-distillery that is under major threat and a weak Thieves Guild.
My favourite clan is the Gray-Manes, not because their pro-Stormcloaks but because they are willing to attack a Thalmor Fort (with just the 3 of them)....... that's just badass.
My most Hated Clan is between the Silver-Bloods and the Black-Briars. The SIlver-Bloods because of their use of Cidnha Mine to make money and imprison enemies, their use of the City Guard to stop any outside interference and their controlled usage of the Forsworn. I hate the Black-Briars for their arrogance, their control of the Thieves Guild and the City Guard to destroy her enemies and their (Maven's) ties to the Thalmor. Considering everything I hate the Black-Briars more since Sibbi is a murderer, Ingun and Hemming are brats and Maven is a [censored
] (plus I don't really mind the Silver-Bloods exploitation of the Forsworn).The Most Corrupt clan is again between the Silver-Bloods and the Black-Briars and again I must choose the Black-Briars, mainly for their links to the Thieves Guild, Thalmor and the Empire which Maven uses to her advantage every time.
EDIT: Feel free to comment why you chose what you chose

Accidently deleted poll sorry... No idea how..
