In the beta my favorite would have to be the hive seeing as to how the Wizard is more of a problem than a captain seeing as how a wizard is more mobile than the captain since all the captain can do is teleport while the wizard can fly shoot and create a field that slows and damages you, the knight is heavily armored and can kill you quickly with the hive boomer or sword, the acolytes can take you one from a range or also have a hive boomer, the ogres can take a huge beating and can take you up close and from a distance and the melee guys (forgot what they're called) can be very dangerous in a group. but in the full game I'd probably have to go with the Vex seeing how a headshot can't kill them and with the twist of them still being a bit organic even though the vex want to wipe all organic life forms.