How do you get the best sword in oblivion

If you don't mind casting spells or exploiting a glitch then the weapon with the most base damage is the claymore from the bound sword spell.

also if you don't want to cast spells a lot then summon the claymore get it damaged(it doesn't matter how much just get it damaged) repair it(make sure you repair it yourself, don't go to a smith) and now it can be dropped
drop it, wait for the timer to go away or dispel it and it will still be on the ground pick it up and it's yours forever

this also works with armor
you may be able to enchant it though i am not sure
there realy is no best sword. there are many good weapons out there for you to slay with. search caves and dunguns if you want good gear. i once found a cave and made almost 15k out of the loot from it and im only lvl 9. on playsation network add xcx179 if you have any further questions