If your enemy has no weaknesses, choose Adamantite greatsword.
An enemy doesn't need a
weakness to an element for that weapon to be useful, only no
significant resistances to it. Obviously if they have a weakness that's preferable but it's not essential - unless you're really struggling with that opponent.
Adamantite's thing is that it is purely physical; in the earlier game so too Gurguth's Maul, Bone Maul, Executioner's Axe and a couple more once you hit the Ashen Wastes. They particularly excel in overcoming high armour (especially armour and shield) and VDX mentions above how skills can make them especially valuable.
In addition to Physical, I have a fondness Spirit weapons as nothing has huge elemental resistance to it and you can set-and-forget; it'll be reliable in the frozen wastes, lava pits, undead crypts, etc.; but if you have a big/dangerous raid ahead then it'll be most optimal if you take a lava weapon into the frozen wastes, a chill weapon to the lava pits, a Holy weapon to the crypts, etc.