I think that quite a few of these could be the "best" depending one's specific goals. Strength and flexibility, as far as I have been able to achieve (limited on the flexibility and much better in the strength department) may be best addressed by different disciplines. I do a combo of Pilates and weights once a week with a trainer (have for 10 years) and the increase over time in my overall strength level (including core) is remarkable.
In almost the same period of time one of my grown sons has taken up yoga, particularly Bikram yoga, and his flexibility at 34 is quantum leaps above what it was even when he was in his teens. So it depends what you're looking for. I am now adding some flexibility work to my weekly schedule but it certainly can't be achieved by the Pilates which has made me stronger than most people my age. So I'll do enough Pilates and weight work to maintain my strength level and put in some yoga (but I don't think Bikram - I couldn't handle that).
If you had to pick just one as opposed to cross-training then I think ballet is probably closest to perfect in that you have to develop core strength, flexibility, grace and balance to be any good. Ballet will not really address cardio and endurance and that comes into play if you are competing with challenging programs.