It partially depends on where you live.
If you're in the US, as your profile states, then Allen-Bradley might be a good place to start. They've got about a 50 to 60% market share here (I think), which means that you've got a 50-60% chance of running into one of them in the field.
But it's still a crapshoot. You can learn everything about the AB SLC, only to find the only companies in your area that are hiring only have AB Controllogix, which think very differently than SLCs (for starters, no datatable). Or that they have need of someone with Omron experience.
And that's the key work - experience. I wouldn't hire someone to program a PLC if all they know about it is what they've read in the manual. You've got to KNOW how it interacts with the real world, what kinds of problems you are going to run into. Timing issues, scan races, noise, interactions, and a whole host of other issues that any programmer/engineer/maintenance worker has to know in his heart, not just his head. to earn his keep, and keep his job.
Your best bet is to find someplace that you want to work, seems willing to hire someome with limited experience, and then find out what kind of PLC(s) they use, and learn those. Hopefully you can impress them with your desire for self-improvement that they will be willing to take a chance on you.
In the meantime, buy yourself an inexpensive PLC (search this site for threads on some recommendations) and play with it. Do something interesting with it so that you will be able to say that you have experience in programming.
