SCJP6 is the most recent, but there is a new exam (SCJP Plus) coming out. At the moment it is just in beat and no one really knows when it will be released.
Also, be aware that the SCJP 5 and 6 have changed.
If it were me, and I needed to take the exam soon, I'd go for SCJP6; I see no point in sitting the SCJP5 as that is certain to be retired very soon. It is an old version of Java
and Java 7 is being released next year.
SCJP 6 Changes
SCJP 5 Changes