Darth Sidious

Ah, give me the good old days, before we saw Palpatine wielding a lightsaber in Episode III, and his miniatures game representations were killer support pieces instead of mediocre, overpriced beatsticks. The original Darth Sidious is a terrific example of that bygone era: a character that can't do a lot of damage on his own, but sure makes your squad work better.

Sidious has the distinction of being one of the few support pieces who helps out Uniques and non-Uniques differently. He can share his constantly renewing Force Points with one Unique in your squad (meaning, for example, that Darth Maul can Rage as many times as he needs to, or Dooku can use Sith Lightning as often as he likes). He can also get more mileage out of your non-Uniques, granting them a full extra turn without the disadvantages of Dominate. A character taking a Pawn of the Dark Side turn can move and doesn't have to save against the effect, making it strictly better than Dominate. Of course, Pawn also damages the target, but if you pick something like the Droid Starfighter in Walking Mode with Damage Reduction 10, you'll never notice the down side.

Sidious' Commander Effect is cute, but nothing to count on. Since it only works on Living non-Uniques, the Separatists are poorly placed to make any use of it. Teamed with Asajj Ventress, you can double the percentage chance of getting that critical hit, but the odds still aren't great.
Finally, while he's mostly a behind-the-scenes schemer, Sidious can get involved in the fight a bit. 130 hit points take a little while to chew through, and his Sith Lightning can let him contribute meaningfully to the fight.

All in all, the original Darth Sidious is a great piece, and probably the only one that belongs in both droid-centered and Sith-centered Separatist squads.

Overall rating in 100: 3.5
Overall rating in 200: 4.5