Do Shattered Glass characters count? For favorite Decepticon cassette character, i'm thinking SG Ravage or SG Slugfest.
If SG characters are ineligible... Glit, i think. Or maybe cartoon Rumble/Frenzy (personality-wise, the two seemed nigh identical from what i can recall, Rumble just got more screen time).
Toys? i've personally handled a G1 RIRFIB Rumble, G1 Ravage, G1 Laserbeak, and G1 Overkill, and seen a G1 Slugfest; the others i've only seen in pictures, but i think my favorite toy would have to be one of the Masterpieces, but tough to decide which. CHUG Ravage
and Beastbox might be runners up. World's Smallest Transforming Transformers Ravage
deserves a mention, although i'm not sure how honorable it would be. i imagine the Alternators Jaguar
and Battle Ravage
are disqualified for not being cassettes? else i might have to reconsider.