Emperor Palpatine picked a fight with the wrong Matrix-bearer. As Luke once tried to tell him, overconfidence can be weakness - all the smooth-talkin', executive trickery, and Sith lightning in the galaxy don't rate you a point against five million years' experience in rearranging the teeth of a guy with a f***ing fusion cannon on his arm.
I would have stayed away from matching TF Classics up with SWTFs, except now Classics is getting its own TFCC storyline, so I decided what-the-hey. You'll notice, BTW, that the victor here has been nicked and scratched up the yazoo, while Mr. "UNLIMITED POWERRR!!!" has gotten his chest smashed in and one hand taken off for his trouble.
(As to how Sidious here ended up facing off with Optimus to start with...well, I don't ever think of those things, except to say given the former's acquired taste for jumping bodies in the early Dark Horse comics, somehow his corrupted midichlorian self probably got plucked out of oblivion by Unicron, or something...)
Incidentally, I wasn't thinking about it at the time, but this pic probably had some subconscious influence from a climactic panel in issue 4 of Marvel's TF comic - apart from the fact that SWTF Palpatine's robot-mode colors (gray helmet, white body) mirror those of Marvel's Megatron, the panel in question portrayed Megs hefting a maimed Optimus up over his head in triumph. I guess Optimus is getting even to a degree, here.