Stealth scout, which might or might not combined with a ranged role. Not melee/assassin because you might get too involved with assassination which is a solo play. Stealth, armor, then attack.
Meatshield. A full armor melee fighter. This one stand between the squishy members and the oncoming wave of enemies. HP, Armor, THEN Attack.
A ranged sniper. The role is dedicating to remove troublesome targets from afar. Often get consused with scout, but its position and abilities are different. Attack, then HP and armor.
A weapon of mass destruction. Usually it's a wizard/witch focus on firepower, but in SF setting it would be a heavy weapon user (grenade launcher, rocket launcher, mortar, LAM, etc). Pure Attack.
That is a four unarguable members of a party. A fighting party. Any more depend on the games.
If the game has some interesting mechanic, there would be more.
A charismatic leader, if charisma is in play, who might get oddjobs.
A thief-class utilityman who focus on utility aspect like item craft, lock pick, etc...
6 is about the largest size we can reasonably use fully. 7 might tempt us into separating two teams and attempt some fine tactics like hit and feint or roll the flanks. And extraneous characters from those 6, of course.