Gotta be Frieza, he's the only one with a consistent history of fighting when he feels threatened, and if his purpose is to protect me, that should qualify under "feels threatened."
Buu literally missed out on an entire tournament because he fell asleep/didn't care enough to complete an assessment/whatever. Vegeta is a consistent underachiever who mainly cares about himself. Cell feels like the weakest in outright power.
Frieza is pretty much the only one who has, consistently, shown the ability and the drive to over-perform, as long as he has personal stake.
Seplito
Nash, Smelling Like the Vault since 1996
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