I think the coolest thing I could remember having for a final boss type was this gigantic robot I had. It could kind of transform into this huge tank, but it also had treads, machine gun hands and these cool eyes, one of which was like the scouter thing Vegeta wore on DBZ.
It'd shoot lasers, and by "lasers", I mean "My Imagination".
It was pretty cool.
Even cooler were the people that were fighting him (I played with my action figures like an RPG), so I had Bob from Reboot, The Blue Power Ranger that I made out to be the Ninja from Metal Gear Solid (He was missing an arm, felt appropriate), and Wolverine, circa the Hugh Jackman version from the first X-Men Movie.
And the best weapon was the Zeus Cannon from FF: Spirits Within.
Imagination......Imagi...na...tion...[/South Park Reference]