That kind of question is very subjective - you'd have to define the term 'best' before you could even begin to answer the question.
Even for a low transaction system, what kind of storage is available. Is it local or SAN? How large are the databases? How large are they going to get? Is tempdb used a lot?
For most of my smaller systems, I usually start with two drives for the system. A system DB drive sized at about 5GB and a data files drive sized to what is the expected size of the system. When I install SQL Server, I select the Advanced button and change the data file location to the system drive. After the install, I modify the default data/log locations to the data files drive so new databases are created there.
What RAID levels are used will be dependent upon what storage is available (local/SAN), the type of SAN, etc...