You don't know, but it is, by definition, the best thing that could possibly happen to you as the result of believing a lie, as you define 'best thing'.

If that's what you meant, then the choice is really "best thing in life" or "worst thing in life"; whatever belief leads you there is of little consequence. Say the truth option leads to an erudite you eradicating all present, past, and future sentient life, and the falsehood option leads to an ignorant you stumbling upon the nirvana-space that grants all infinite super-intelligent bliss and Dr. Manhattan-like superpowers (ironically enough):

What you believed is of little consequence to the resulting state of the verse(s).