Well, Mystery Dungeon doesn't actually use the gender system. The exceptions are you, your partner (Who supposedly is the same gender as you) and the Nidoran family. That said, the best Pokemon if you've beeten the story already IMO are yourself/your partner and Absol. Or you could use Lati_s depending on whether you're recruiting or not. For recruiting, use Latias. For hard missions, use Latios.