I'd say 5 Luck. Anything above 7 Luck and you can't lose... Unless you do something stupid like hit on 20 or stay on 15.
LOL. No, trust me, I have 7 Luck and I doubled down on everything. 20s included. I'd just get an ace. At 7 Luck if you double down on everything, max bet, you will win around 3 or 4 out 5 hands no matter how absurd it is. You'll also get blackjack every few hands. In about 15 minutes you'll break the casino. I did it to all of them last night. I haven't tried it, but another person on this board played with 9 Luck plus sixy Sleepwear (effectively 10 Luck) and didn't lose a single hand.
Another person mentioned they were having no luck at 6 Luck. They got an implant for 7 and instantly started experiencing what I described. So apparently 7 is some magical cutoff point.