What the strongest sword

Well that depends. Some would say the Badelaire for it grows in strength as you explore. Others insist it's the Sword Familiar at lvl50 as you can wield it as a sword. Personally I use a combination of Mourneblade (dropped by the Azghal monster in the INverted Castle's Collisseum) and Crissaegrim (dropped by the Inverted Library's Schmoo creature)

Here's why: Crissaegrim dishes out insane hits to the target creature, killing most everything in just one to two presses of the button. Mourneblade works like Soul Shield, you get back 8HP per hit with the sword so in a sense you have infinite life and insane damage potential.

Hope that helps