From NWNWiki

The fortitude saving throw measures the ability to stand up to massive physical punishment, such as poison, paralysis and instant death magic. The constitution modifier is added to fortitude saves.

The classes with high fortitude saving throws are

• Arcane Archer,
• barbarian,
• blackguard,
• champion of Torm,
• cleric,
• druid,
• Dwarven Defender,
• fighter,
• monk,
• paladin,
• Pale Master,
• ranger,
• red dragon disciple, and
• shifter.