There's no such thing as a 100% chance of turning purple. Your environment must be able to drop temps at night by around 10 degrees from the lights on temp. And then, once you have that done, which is a task all its own, you can try strains like Purple Erkle, Purple Kush, Blackwater, Grapes, Lavender, Purple Mango, Purple Cadillac... the list goes on. Research is in order. If you get a clone, you can simply ask if the person or dispensary bud tender has grown it out and what helps that strain purple the colder temps are enough. If they're selling healthy clones, theres a good chance you can trust they know how to grow well and the information they give is valid, to a certain extent. Also check out seed sites for general information on strains.
And "best" is purely a matter of opinion. As of now, Purple Mango and Purple Erkle are my favorite indicas. Erkle hits you hard but you can still function, make coherent thoughts, and express them in a clear manner. Purple Mango is better imo becuase it too hits hard, but it also smells like mango. I LOVE fruit flavors and produce, but some would rather have a spicy flavor when smoking, or creamy, or earthy.... you see where im going?
Best of luck.
P.S. I've grown Purple Cadillac under far less than ideal conditions and it turned purple for me. It wouldve turned more so if i let it go for longer but thats another