Here is the order I would recommend: Daka to 4*, Ahsoka to 5*, Fives, then Scavenger (possibly sooner depending on how farming the other Jawas is going).
If you are having trouble with DS missions then I would recommend activating Daka (4* only). Her multi-stun basic and heal special helps you survive longer, until you can unlock and gear up stronger DS toons. Additionally, she makes the Support/Healer challenge much easier and is useful in GW. There is gear that is DS only (e.g. Mk 5 Stun guns, Mk3 Chedak Comlink).
Ahsoka by herself is good for Yoda challenge and ships. Her usefulness in other areas of the game (AAT raid/GW) depends on having JKA.
Fives is great for pit raid, but Phasma will work in his place for inflicting speed down. Unless you are going for a clone team (which is a long grind), there are more useful toons to target.
Scavenger eventually needed for crit damage mod challenge. Unless your other Jawas are up to snuff with gear/stars/levels though to attempt the mod challenge, he becomes less important. Useful on AAT raid for p3.