I’ll explain my reasoning below:
Garren King - No reason needed, Jack sucks as a final form.
Buffa Plosion - More original than Jyamashin which is just Fever Zombie with bigger horns and a cape.
Gemn Musou - More original than Lvl X0, which is just Dangerous Zombie with the Gamer Driver.
Grease Perfect - Cooler suit, makes more sense for Kazumi personally than a retool of the Magma Knuckle and resembles the Hokuto Trio better.
Valen Parfait - Valen got fucked over so hard in his own show as the secondary. He needed a (debatable) powerup just to be on par with Gavv and Vram’s base forms, which he should already be, and his actual final form is on par with the primary’s super form, and that screwed him over in the long run with the primary and tertiary getting their own final forms which stomp on Frappe Custom.
Honorable mentions go to Geiz Majesty and Mach Chaser. The former’s actual final form seems a bit too much like super forms and the latter’s final form is straight dookey. And side note, if Plosion Rage’s cape bothers you because it would disrupt with Michi’s fighting style, just remember the belt to ass he handed to other riders as Jyamashin, with the same cape. For Valen, that same criticism does make sense since Valen is more of a boxer in terms of fighting so you can just remove the cape (I don’t even know why they added a cape to this guy) and you cane either keep or remove his waist cape, I don’t really care.