I'd rephrase that question to "what's the best gmat prep course for you?".
There are a few factors that will influence that:
1. Your learning style -- do you need the energy of a human element (classes or coaching) or are you able to stay motivated with an online learning tool?
2. Your exact starting point with respect to quant, verbal and DI. If your conceptual foundation is already strong, then less personalised solutions like online tools or group classes can be less effective. You need to focus your studies on your specific gaps, which a "one-size-fits-all" solution won't do.
3. Your target score -- the higher your target score, the more likely you will need some personalised coaching (1 on 1). This is to precisely target and break through your remaining gaps to perform at your peak.
It's no joke to reach the top percentiles of the GMAT Focus -- choosing a path (class, coaching or self driven) is the critical first step to building momentum to that goal.