Which banner to pull Jack the Ripper?

Wait for her summoning banner as she's the only SSR on rate up and is not with Jeanne meaning higher chances of getting her.

Wish I could want her as much as you do, got np2 of her today rolling for acherturia and the first one on the gssr. Would have traded her for any other 5 star that I don't have, but back on topic the best time to roll for her is the christmas rerun banner since she's the only 5 star on rateup.

You'll have a 50% chance of getting her in the next guaranteed gacha, unless you're looking for other SSRs as well (you'll have only one roll).
http://fategrandorder.wikia.com/wiki/2017_New_Year_Lucky_Bag_Summoning_Campaign#Assassin+EXTRA