Sableye: The most efficient thief you've ever seen. One quick Trick and your jewels or other items of value have been replaced with an apple core or some other garbage. Whatever I have lying around in my pockets, I guess.
Gengar: Stealth missions. This dude literally dissolves its body into shadows, so it has stealth capabilities that are far superior to any ninja. This is advantageous for thefts, infiltrating buildings (Gengar just dissolves and passes through barriers, and is then able to let the rest of us in), strong-arm tactics (how are you going to fight back against your own shadow?), and spying.
Zoroark: To conceal our hideout and to act as a spy/decoy, Mystique-style
Ditto: Similar to Zoroark, but it seems Ditto are better at concealing themselves as objects as well as people. Could definitely be handy to have a henchman that can pretend to be the gun some idiot police officer or rival criminal thinks they're going to shoot me with.
Rotom: Let's face it, I know nothing about how to steal cars, and if I'm turning to a life of crime, then I probably don't have enough money (yet!) to afford a series of getaway cars and the like of my own. With Rotom, however, I can just look at any vehicle with a motor I fancy, be it a car, a riding lawnmower, a tank, a helicopter, an airplane, or Bowser's flying clown machine, ask Rotom to possess it for me, and voila! I've just acquired some free transportation! Best of all, possessing it means I don't have to use gasoline, and thus I'm not polluting the environment. That will be a point in my favour if I ever get hauled into court.
Eelektross: Well, every criminal needs some form of intimidation, and so far, the most intimidating thing I have is a smiling gas-bag and a bipedal fox. Scary, but not the sort of thing that makes people tremble in fear and lose the will to ever challenge you. That's where Eelektross comes in. What's so scary about Eelektross? Check out that sucker mouth. They're based on real-life creatures called Lampreys, and specifically carnivorous lampreys. That mouth really isn't much good for traditional biting. What that kind of mouth does is slowly cut into the victim's skin until it finds some fluid to drink. So, sure, you keep your gun (which is actually my Ditto) and try to threaten me with it while dodging dark energy that is shooting out from your own shadow. I'll just order my mutant lamprey to bore a hole in you. There may be a bit of a mess to clean up, but I'll just have Zoroark create an illusion of empty space where your mangled, drained out body is so no one will ever find out what happened.
Or you could just run and not bother me. That's probably a better choice.