This may be the only time you'll see me in this section, but that's besides the point.
Anyways, I'd say Barry, because considering Serena is 12-ish in Gen VI, that would make Gary roughly 7 years older than her, and the only a person is 7 years older than the person that they're with is when they're adults...But the again, by that logic, Barry would be 5 years older than Serena, too...But, if age isn't a factor, I'd say Gary because he's not as crazy as Barry, even though his maturity may be more comparable to Barry...Hope this helped!