Im a little late on this, been having a rough time coming up with stuff to post lately...
When I first read the question from Ron, my first thought for my Red Scorpions was Space Marine Tactical Squads. Why not? They are flexible and capable in most roles, and they count as a scoring unit! Even if they never do anything spectacular, they are always there for me.
Then I got to thinking, and actually looked at the army I have and use, and I have to admit, my Must Have unit is actually the Assault Squad. With a trio of plasma pistols (two on troops, one on the Sarge) and a Power Fist, these guys can lay down a nice beating when up in your face. Add in a speedy move and 30 attack dice on the charge, and you have the foundation for a really brutal squad.
"What do you mean, a foundation?" Hehehe... One unit, no matter how "Must Have" does not fight alone. What makes the Assault Squad my Must Have unit is that they usually act as an escort for another model. Think a fast unit with melee weapons, a few high power shots, and a Power Fist is nice? Add in an HQ choice with a jump pack and watch that unit SHINE!
A Librarian with a Force Weapon can take advantage of the numbers for protection when hunting the larger targets, and the unit can always use a few extra Assault based shots.
A Chaplain will make the unit a lot more potent when dealing with a mass of lesser foes, though being Fearless can potentially screw you, especially in instances where you actually want to use Combat Tactics.
Finally, the Captain is useful for being an all-rounder, though with wargear he could potentially cost more then the assault squad on his own!
Lately, Ive been messing with not deepstriking, but having the unit spend the first two turns or so hanging out behind cover, then using their speed to smash enemy units that have braved my bolter fire and gotten too close.