For maps where infantry are dominant medic is the best class; so in IO it's medic, period, if you have to pick just a single class. If you're a good sniper (not a good shot, a good sniper) then that can be worthwhile too but medics make more of a difference in winning or losing as a rule... if they're doing their job, getting all those lost tickets back.
For any map featuring armour then engineer is the class for those in the armour. AT has something to be said for it since you can hop out and shoot at enemy vehicles yourself, but you should be able to repair your own vehicle for long-term effectiveness.
For any map featuring armour then engineer is the class for those in the armour. AT has something to be said for it since you can hop out and shoot at enemy vehicles yourself, but you should be able to repair your own vehicle for long-term effectiveness.