These provide the most protection while remaining within the weight class.
These values are without a shield.
Each type of enemy is more vulnerable to certain damage types, and generally they are most vulnerable to at least one elemental damage type.
You can convert a portion of your weapon damage to an elemental damage type using a gem in your weapon, up to 50% with a pristine gem, however the benefit is not as great as it may seem for melee weapons.
The 100 Strength perk gives a +10% damage increase to physical attacks.
If you convert a portion of your damage to elemental, that portion no longer benefits from this perk.
So, for example, if you use a Pristine Topaz to convert 50% of your hatchet to lightning, it will appear that it should provide a net +15% damage increase against Ancients. Against Ancients slash will deal no extra damage, but lightning will deal an extra 30%, so with half of your damage converted to Lightning, that would appear to provide a net +15% damage increase.
However, due to the Strength attribute perk at 100 Strength your physical attacks gain +10%, which the elemental side of the damage does not benefit from.
Therefore, you only effectively get a +10% net damage increase. If you consider that gems like Diamond or Opal can give you a flat +15% increase (if you meet their requirements), using an elemental gem can actually lead to less overall DPS.
The advantage to the elemental conversion is that it is a flat predictable amount.
This does mean that there are several cases that converting to elemental damage will generally give an actual damage decrease, assuming you have the required attribute perks, so you should never use these:
As an additional note, this will not be as true for Rapier, which does scale with Intelligence, and the 100 and 150 Intelligence perks provide extra damage for elemental damage. Therefore, it could be better in more cases to use a elemental conversion gem on Rapier specifically, as long as you are hitting those Intelligence thresholds.