It has been a couple years since I created a new ControlLogix project, and apparently we are now on version 28!
This is very much a summarized version and from memory so I may have forgotten something, if so I apologize:
V20: Last Version of RsLogix before the Transition to Studio 5000. Last Version to support L6x ControlLogix controllers and the L23x, L31, L32x and L35x CompactLogix Controllers. There are 3 versions of V20, V20.00, V20.03 and V20.04. Use V20.04 if you are using version 20.
V21: Introduced Studio 5000 and support for the Kinetix 5500 servo drive. Note there are two incompatible versions of V21, V21.00 and V21.03. You can move from V21.00 to V21.03 but not back. The change was a security issue.
V22: Does not Exits, only an internal Release within Rockwell.
V23: Basically released to solve the incompatibility problem of V21.00 and V21.03.
V24: Lots of new features including Local Parameters that can be shared between programs, Compare and Merge Tools, Logical Organizer etc.
V25: Does not Exits, only and internal release within Rockwell.
V26: Introduced support for Kinetix 5700 servo drive & Licence based Source Protection.
V27: Introduced View Designer to work with the PanelView 5500.
V28: Introduced support for the L8x series of processors.
If you are going to do a new project then I would use V24 or newer. The new features in Version 24 are very good. Unless you need to use them for the new Hardware support I don't see much advantage in versions 26-28.
Regards
Ian McDonald