Siemens Simatic

Siemens is very large with a very large scope of operation, as such they have good market reach.  This means good documentation and usage in the field so if you have a problem it typically isn't too hard to find the solution.  That being said they make it very hard to even get a look at their tech if you are looking to expand your knowledge or do some basic research.  Try downloading a trial version of their development software and you'll know what I mean.  Here are their current main offerings for PLCs:

• Simatic S7-1200 - Compact/Small "Basic controllers are the intelligent choice for compact automation solutions with integrated communication and technology functions. They are available in standard and failsafe versions."
• Simatic S7-1500 - Advanced "You’ll achieve the highest productivity and product quality in your production process by using SIMATIC S7-1500 with its fast backplane bus, PROFINET performance, shortest reaction times, and a command processing time of up to 1 ns in the CPU."
• Simatic S7-300 - Advanced "The SIMATIC S7-300 universal Controllers saves on installation space and features a modular design. A wide range of modules can be used to expand the system centrally or to create decentralized structures according to the task at hand, and facilitates a cost-effective stock of spare parts. SIMATIC is known for continuity and quality."
• Simatic S7-400 - Advanced "Part of the SIMATIC process controller series, the S7-400 has been designed for system solutions in the fields of manufacturing and process automation. This process controller is ideal for data-intensive tasks that are especially typical for the process industry. High processing speeds and deterministic response times ensure short machine cycle times on high-speed machines in the manufacturing industry."