Selecting a Virtual Learning Environment is complex and depends on what is important to you. It is a bit like asking "What is the best car", the answer will depend on what is important to you. I know that is not directly helpful, but if you can draw up a list of the most important issues then you can be advised on what is the best system. However I can shortcut your enquiry by saying the answer is Moodle (joke....... partly)
Moodle research
Learning System Benchmark References
Re: Learning System Benchmark References