SPOILER WARNING
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Here are my picks:
Colonel Saul Tigh: Simply for TV drama and effect. Arguably the one that hates Cylons the most, and hates himself at the same time. Coincidence? I think not.
Petty Officer Anastasia Dualla: She is in competition with Starbuck for Lee's heart. Starbuck is probably the Chosen One, so interesting symmetry. And she's too cute to be human.
Lt. Felix Gaeta: Always in a position close to those in power. Maybe his machine-like efficiency (and affinity with machines) extends beyond appearances...
Tom Zarek: The guy you love to hate. And an ambitious cold-hearted troublemaker. Maybe too obvious, or maybe not.
Oracle Selloi: Beirs was surprised by one of the Five, and recognized her with an incredulous "You!". The Oracle did a good job of spooking her on New Caprica...
Your turn.