Silvers are the best way to go. Bronze packs will give you a LOT of very common items... good for building a good stash of low-value items for Warzone matches, but won't result in many permanent unlocks proportional to the RP spent.
Silver packs are your best value... 2 guaranteed permanent unlocks, and all but the most rare card types. Because the game will actually take your current inventory into consideration, building your permanent unlock list with silvers first will eliminate most of the lower-level permanent unlocks as you go. Later you can use Gold packs, with only the rarer cards remaining. This will virtually guarantee Golds will result in the rarest cards, and that you won't waste 5000 RP on low-level unlocks.
So if you want low-level weapons and vehicles to keep you supplied in Warzone... buy a few Bronze first... then focus on little but Silver unless you need more stock than Silver is granting you (then pick up an occasional Bronze, which should now have a better selection of cards thanks to your silvers). Only buy Gold once you've unlocked damn near everything you can with Silver.