The best weapons in Fable and Fable the Lost Chapters are the Solus Greatsword (due to its damage rating) found in Bowerstone North once the Arena quest has been completed, or the Sword of Aeons due to its speed. The Sword of Aeons is the most powerful and easily the best weapon in Fable; however its damage rating is lowered significantly in The Lost Chapters, making the Solus a more powerful weapon. The Solus costs around 50,000 gold, so it's not so easily acquired. One method of getting it is to marry Lady Grey and treat her nicely - she'll eventually give you the sword.
Skorm's Bow and Arken's Crossbow are the two most powerful ranged weapons in Fable and the Lost Chapters. Skorm's Bow is difficult to get a hold of because it requirs a lot of sacrifices at the Temple of Skorm in Darkwood. To boost the chances of getting Skorm's Bow, get your alignment in the 'good' range, and sacrifice the good mercenaries you can recruit in Oakfvale just outside the Memorial Cemetary entrance and outside the Temple of Avo in Witchwood. Arken's Crossbow is much easier: it's located in the 15 silver key chest in Darkwood.