Depends on where your stats are currently sitting.
If you're a fresh-faced scrub with no stats higher than a 3, there's only 2 jobs you can do anyway: Roxy's Diner and the Gentlemen's Club.
Roxy's Diner pays quite poorly and trains speech (an almost useless stat), and can only be done 4 times per day. The Gentlemen's Club can be done as much as you have the stamina for it, but at a cost of 10 per session, you should only do it once or twice to get you some starting cash, so you can then enter New Ark.
Once you got your strength to at least 5, you can head to The Rock and talk to the man behind the bar to work as a bouncer. Pay is pretty decent and will also train strength further.
If you get your physical stats to a respectable balance (strength, nimble, and resistance averaging 10 or more), you can head back to the slums and give food to the hungry hobo every day in exchange for tips. Eventually he tells you the password to the underground arena in the slum's bar. Winning fights here is quick and easy money, but with the risk that Lin Lin might show up to fight you, and she is a formidable opponent. You don't lose anything for being defeated, but you can only attempt the fights 10 times a day.
For the long-term, by far the best at purely making money is strip teases in the Gentlemen's Club, but it's not fully worth it until you have respectably high stats in Res and Ero.
Res because that's where your max stamina comes from, and Ero because that's what affects the max money you can make.
With 16 or higher in both, and something to help you spam through the dialogue, you can easily make several thousand per minute.
As an added bonus, the location is very close to Lin Lin's massage parlor, who can get you all your stamina back for just 15 credits.