I've always like the Ghost Faced Killer (Complete Adventurer), but the implementation is weak. I do like the picture as well:
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Mind Bender (Complete Arcane), is actually a pretty good one-level class, but the other nine levels suck. I love the look on the guard's faces:
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Pious Templar (Complete Divine), has a cool picture (I love most of Wayne Reynolds' work):
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Same for Sacred Fist (Complete Divine). Another Wayne Reynolds:
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