Fast Blade / Daggers / Light Swords: Lightning Rune is your best pick. These weapons hit frequently, which maximises Lightning's chain lightning procs. Second pick: Shadow Rune if you have crit from gear. Avoid: Fire Rune — the burn ticks are too slow for your attack rhythm.
Heavy Sword / Greatsword / Hammer: Fire Rune or Shadow Rune. Both reward slow, deliberate hits. Fire adds sustained burn damage between your swings. Shadow makes your critical strikes hit even harder. Second pick: Earth Rune for extra survivability when learning new dungeons. Avoid: Lightning Rune — your attack speed is too low to make chain lightning worth it.
Staff / Magic Weapons: Shadow Rune (late game) or Lightning Rune (farming). Staff builds deal most of their damage through skills, so Shadow's crit boost amplifies your burst windows. Lightning's chain lightning helps clear groups faster during farming runs. Avoid: Earth or Life, unless you are still learning.
Spear / Polearms: Lightning Rune or Ice Rune. Spears have good reach and moderate speed — Lightning gives you wave clear, Ice gives you the control to keep enemies at your ideal range. Neither is strictly better; it depends on whether you value speed or safety more.
Shield / Tank Builds: Earth Rune. Your job is to survive and hold aggro. Earth's flat damage reduction scales with your existing defense stats. Second pick: Life Rune for sustain if you have a healer in your party.
Hybrid / Unsure Builds: Lightning Rune is the safest all-rounder. Its chain lightning adds clear speed regardless of your exact weapon, and the attack speed bonus benefits most builds to some degree.