Well, I haven't GM'd much at all but the worst thing on the player end a GM has unleashed on us by far...
Emperor Palpatine.
And he didn't use the official write-up. This dude was like 96 pointbuy, Draco's uber power from DoD 9 SotE, topped off with a +38 UtF or something ridiculous.
total TPK in like 7 rounds
Maybe I'm just getting soft in my old age, but that seems kinda uncool to TPK the party with a way way over-the-top homebrew version of Palpatine...even if it was a campaign ender.
Now if the PC's did willingly choose (and were not led by the GM) to go after Palpatine, and choose to go try and fight/kill him, I wouldn't feel bad TPK-ing them.
I'd probably use something a little more "by the book" than that thou (IMO that would "feel" better to the players, even if they lost). As player, I'd feel kinda cheated by a uber homebrew creation and loss like that. Palpatine has the Visions talent, so he really doesn't need all that. He could foresee the PC's coming attack, prepare, have Vader and the 501st there, with Veteran Imperial Officers with the "Trust" talent (for extra standard actions), etc. Either that, or just lay a trap and have them all killed in a massive explosion, or whatever. Just my opinion
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Sparx MacGyver, cool AT-WF walker! I like it.

The heavy turbo lasers make it a vehicle only fight (or PC's with Improved Cover or Evasion). One shot like that and its game over for most PCs.
YIDM