Statistically in KOTOR 2 which is the weakest prestige class

I'll let you choose.

This is the basic stats for each prestige class, which you can find in the KOTOR 2 manual.

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Jedi Weapon Master/Sith Marauder: Vitality Points/Level-10; Force Points/Level-6; Skill Progression-Slow; Feat Progression-Fast; Force Power Progression-Average
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Jedi Master/Sith Lord: Vitality Points/Level-6; Force Points/Level-10; Skill Progression-Slow; Feat Progression-Slow; Force Power Progression-Fast
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Jedi Watchman/Sith Assassin: Vitality Points/Level-8; Force Points/Level-8 Skill Progression-Average; Feat Progression-Average; Force Power Progression-Fast
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It's been said that the best combination for Class and Prestige is Jedi Consular/Jedi Weapon Master. But it really depends on what you want to do.