The best way to invest in Bitcoin is to understand the market movements, and place a pending order price when Bitcoin is dipped it automatically executed as a spot trader and sell when when return back to original stage make a little profits and it can as well by holding as investment for a period of time holding. The major things that matter in Bitcoin investment is your entry point and capital used to buy, that determine you're profits return investment.
Since you brought this topic back to life, I can add my 5 cents...
Well, from personal experience, best way to invest is the legal way you have in your country through the exchange you have. So, OTC or any shady (if that's the question) is bad, you definitely need to make KYC and must buy through legal way for later eventual tax payment.
When is the best timing it's something else (if that's the topic). Usually, best time to invest in bitcoin is when everyone and everywhere are bearish - meaning, they are not optimistic, they are negative, there is a lot of negative news etc. There is also fear & greed index (google it), when it drops below 20, usually is the best time to buy.
So, yeah, if we keep it simple - buy when others are in fear and think everything will drop to zero.
But also before buying you need to set your goals straight, otherwise you will lose it eventually.