I think its made fairly apparent from the beginning of Garion's evolution into Belgarion that he is without question the most powerful. The Orb of Aldur certainly lends him tremendous strength, but without it, Garion was doing things that Belgarath could never have. Creating LIFE with Horse for instance. Belgarath would have died trying. Garion succeeded.
I think its hard to rank the others, since they rarely faced eachother. Belgarath certainly had it all over Zedar... yet, Zedar didn't really try all that much... he was overcome with remorse... and dread at having finally committed the one unforgivable act of his eons long life.
((side-note... Zedar is perhaps, IMO, one of, if not the most important character in the books aside from Belgarion. He DOES the most things, aside from perhaps Belgarath...its not a stretch to imagine Zedar completing all the prophecised tasks in Mallorea whilst Belgarath was busy in The West.. and Mallorea is infinitely bigger that just the West. throw in his "finding" of Erriond, his crucial theft of the Orb, and the absolutely pivotal killing of Durnik... who for anyone paying attention was in no real danger of a heroic death... in fact Durnik was just plain STUPID for attacking Zedar at all- sorry for my pro-Zedar rant, back on topic)).
Belgarath is certainly far more powerful than most of the others, Ctuchik was only a challenge due to Belgarath's continually drawing on his will to help protect the party on their way to Rak Cthol. His confident arrogance in regard to the Magician of the Murgos is well founded.
I'd rank Beldin as being Belgarath's equal in all regards...Beldin is stronger physically and also FAR more intelligent, but the strength of their wills i assumed them to be practically equals.
Polgara is not as strong as either. her truest strength is the subtlety of her powers. Having said that, i would put her above every W&W wielder who is not a Disciple of Aldur.
Poledra is even more subtle than Polgara... so ranking her is impossible since we never see her actually do anything except transform into animals, and share Polgara's mind/body at Vo Mimbre.
Beltira and Belkira, indivually would be Polgara's better, but together as they would always be, far stronger... we see the effect of their dual power in the Battle of Thull Mardu... the twins are exhausted but conscious after generating the storm, and then continuing to have to fight all the heirarchs as well. Polgara is out cold.
Belmakor and Belsambar are hard to place since we see them so briefly... from what clues we are given, i would be happy to place them above Beltira and Belkira as individuals, and possibly about as strong as Belzedar (yes BELzedar as he was at the time). Each having particular strengths... Belsambar's unparallelled strength at illusions, Belmakor's brilliance in construction - not to mention his mind that had him debating with Beldin for eons... plus, he was Belgarath's favourite brother...so he get a bonus on his Talent ranking
Urvon and Ctuchik.... Urvon is pathetic, and we are given every indication of his weakness. Certainly for a jumped up Grolim he's awesome, but on an international stage... he's the Comedy Relief. Thanks to Beldin of course, and his white hot hook of terror.
Ctuchik, a threat, i'll give him that. Whilst i think he'd be a true challenge for Polgara; as i mentioned above, it was only Belgarath's exhaustion that gave him a remote chance of defeating the Eternal Man, and he certainly would have lost had the battle continued any longer. Beldin, as well as Belgarath is underwhelmed by Ctuchik's "power" when he met him...
Asharak/Chamdar, the Heirarchs perhaps, Salmissra possibly, Gorim of Ulgo, these would all come into a secondary tier of Talented people. The Grolims in the next below.
The trickiest question of course... is where do the various mystics of Kell. At the very least, on the same level as the Grolims, but probably more powerful. The hive mind thing would make them very formiddable.