The most important option for mercenaries is to steal lives. Mercs need to get enough lifestyle to keep them alive, but this stat is hard to get on normal weapons or body armor, so we want to have it on their helmets.

Even better if you can find someone with stats and resistances. Since there are not many runic helmets and there are no helmets with life-stealing, unique D2 Resurrected Items are most recommended. Since its durability hasn't dropped, ethereal items will be better.

The most popular helmet among mercenaries is Andariel's Visage. Its defense stats are great, it has 20% attack speed, 10% life steal, 25-30 strength, and +2 to all skills. Unfortunately, it reduces fire resistance by 30%, so people made a slot to insert a Ral rune to cover it.

In addition to this, Vampire's Gaze or Rogue's Crown are also used. The set item Tal Rasha's Horadric Crest is also a good option.

Vampire's Gaze has 6-8% life steal and damage reduction options, while Thieves' Crown has 9-12% life steal, +25 agility and +33% fire resistance.

Tal Rasha's Horadric Crest has 10% Life Steal, +60 Life, and +15 Total Resist. If you don't have these items, the Undead Crown or Stealskull are good options. 

Best place to Buy D2R Items - RPGStash

Only relying on the character's accumulation of runes in the game is completely unable to meet the level promotion. You can quickly buy Diablo 2 Resurrected Runewords from reliable suppliers in the market. Generally speaking, game value-added services are quite mature, and you do not need to worry about account security. RPGStash is a game delivery service with nearly 20 years of experience, 100% secure, fast delivery, and 24/7 online support.