Most of the legendaries that you're missing are only worth owning for completion's sake. The only ones I see that are actually playable are Gormok, Black Knight, Geddon, Toshley, and Capt. Greenskin. Out of those, I'd say either Gormok or the Black Knight depending on what types of decks you play. BK is a better fit for control decks, Gormok goes into board flood decks which tend to be aggro. Geddon is pretty niche. He's rarely seen outside of Control Warrior. Toshley has good stats for the cost, but the spare parts are only really useful in Flamewaker decks. I've always felt like Capt. Greenskin has been underrated, but he's really only ever going to be useful to 3 classes: Warrior, Paladin, and Rogue. The other weapon classes (Hunter and Shaman) don't rely on their weapons like those three.
Thanks for the advice!