Life Drain and Mana Drain may be considered magic damage since they are not affected by the target's shielding or armor, but it is currently impossible for players to cause this kind of damage. Note that now most life drain attacks have turned into death damage after the Winter 2007 Update.
Due to the nature of Death Damage, most creatures are not strong or immune to its effects. Fire Damage follows closely behind as the most common type of damage to use against creatures. If neither fire nor death are effective against the creature, Holy Damage will most likely be its weakness. If all three are ineffective then you're most likely up against ice or water type monster in which case Energy Damage is most effective against it.
Note that not all spells and runes cause Magical Damage, some of them cause Physical Damage instead. The only runes that cause physical damage (after Update 8.1) are the Explosion runes. And the only instant spell (after Update 8.7) is the Physical Strike.