i would advise you grab the grenades or one of the player abilities first ... the unlocks are not that better than the stock at the start
then as you play grab the packs from the dead and experiment with the loadouts to see what unlocks you prefer, failing that use the single player game so you can try all the unlocks straight away
the stock assault and sniper rifles are probably still the main guns i use, apart from the wonderful clark 12b .. best gun in the game