It is the willpower attribute assuming it is a mana spell. If it is a physical spell like powerbolt, lightning bolt etc. then it is resisted with the body attribute.
Actually that lightning bolt is resisted with Reaction(the counterspelling adds to dodge test)
Max, you're right, but you're also giving the OP a headache for doing so. . .
So, in review:
Resist Direct Mana Spell:
Mundane: Willpower alone
Mundane protected by a Mage's Counterspelling: Willpower + Counterspelling
Mage: Willpower + Counterspelling
Mage w/Magic Resistance Quality: Willpower + Counterspelling + # Levels of Quality
Mundane w/Magic Resistance Quality: Willpower + # Levels of Quality
Mundane w/Magic Resistance Quality protected by a Mage's Counterspelling: Willpower + # Levels of Quality + Counterspelling
Resist Direct Physical Spell:
Mundane: Body alone
Mundane protected by a Mage's Counterspelling: Body + Counterspelling
Mundane w/Magic Resistance Quality: Body + # Levels of Quality
Mundane w/Magic Resistance Quality protected by a Mage's Counterspelling: Body + # Levels of Quality + Counterspelling
Mage: Body + Counterspelling
Indirect Spell:
Spellcasting check is
opposed (NOT resisted) as a ranged combat check with the option for counterspelling (but not magic resistance) as presented above. See SR4A, pg 204, and 91 - 92, and long-winded discussion
here.