As the vBulletin staff keep developing vBulletin software, it gets better and better. I prefer 3.6.8 myself, as it has no errors or any protection. vBulletin 3.7.0 (RC4+) has many security token errors, but RC3 doesn't and it is good. Well, they both are good but if you don't want extras you should choose 3.6.8. =]