This is a scheduled post while I'm away but please do add your own suggestions in the comments and I'll see them when I get back! With thanks to Penny Goodman for the idea.
In no particular order because it all depends on your sense of humour, but starting with my personal favourite:
'That's what conquering nations do. It's what Caesar did, and he's not going around saying "I came, I conquered, I feel really bad about it!"'
Spike, on why Willow can't make the Native American spirit feel better, in Buffy the Vampire Slayer, 'Pangs'
A messenger brings the news of Lucius Caesar's death to Tiberius and Thrasyllus and they burst out laughing in I, Claudius, 'Waiting in the Wings'
'You lucky bastard!'
Brian is rescued and then dumped back almost where he was by an alien spacecraft, Monty Python's Life of Brian
'Infamy! Infamy! They've all got it in for me!'
Julius Caesar, Carry on Cleo
Flavia, Lupus and Jonathan pretend to have dreamed bad omens so that Captain Geminus can't go on his planned business trip, The Roman Mysteries, 'The Colossus of Rhodes'
'I'm... Spartacus.' 'And so am I!' (3.34 on the video)
The Doctor and Donna, Doctor Who, 'The Fires of Pompeii'
'It vexes me. I'm terribly vexed.' (The comedy is all in the delivery)
'You - are wearing - his -MERCHANDISE!'
'Beware Trojans, they're complete smegheads!'
Lister, Red Dwarf, 'The Inquisitor'
(Sounding exasperated) 'I was at an orgy, Mother'
Octavia, Rome, 'Heroes of the Republic'
And because you can never have enough funny - Honourable mentions:
'Have you been making friends again?'
Crixus to Spartacus, Spartacus: Blood and Sand, 'Great and Unfortunate Things'
'You sold me... queer giraffes.'
(Also sounding exasperated, just after a book has caught fire) 'Xander, don't speak Latin in front of the books'
Giles, Buffy the Vampire Slayer, 'Superstar'