For me personally, I feel like the answer to this question would be half guard/lockdown, for the sole reason that it is the most common guard that I find myself in.
Having a good open guard is also very beneficial, although that encompasses more than 1 guard. X-Guard, Butterfly Guard, Delariva Guard, Spiral Guard, are all types of an open guard.. Hence the reason for Open Guard not being the answer to this question.
So to answer the question, the one guard I would play if I could only play one, would be the guard that I most commonly find myself in.