While it's all RNG luck in my experience buying the Enhanced battlepacks has yield the most success, though this comes more from a position where I like a lot of the blue enhanced skins. The enhanced battlepack also comes with a marginal (but unknown) increase for a bonus item and chance at a legendary so it's possible to get lucky from time to time. How you spend your scrap is ultimately up to you.
You can gamble with the basic battlepacks and hope for the best, you'll be less likely to get something amazing but would have more chances at it due to how cheap they are. Enhanced is a nice balance between the two while finally Superior battlepacks are expensive but guarantee you a legendary, these are best saved for a revision which has legendaries you really want which in my case would be anything medic related.
Finally there's the exchange but usually the prices there are exorbitant at best, I'd only use it if you desperately want any of the skins featured or puzzle pieces and somehow have the 5,000 scrap necessary to buy it.
