#1: Sith Empire Trooper
#2: Darth Maul
...distant #3: Royal Guard
...distant #4:: Sith Trooper.
The Sith Trooper is too valuable to First Order squads to move. I'm not saying he wouldn't take out Geos, just that he's better elsewhere.
The other proposals are non-starters. Attacking Geos (rather than just waiting for them to die) involves dealing AoE damage. If you're not doing AoE, it's a waste. Royal Guard is an acceptable substitute only because he auto-regens when allies get attacked AND auto-taunts, so the fact that his taunt gets dispelled isn't as much of a worry. Since the Geos do lots of damage only by making lots of small attacks, that means that Royal Guard gains lots of TM and heals lots of Health between the times he's taunting. Just re-read the kit and it's not how many attacks, it's just all attacks during one turn that determine healing & TM gain.
Now, one other thing to take into account is that Darth Maul is MUCH easier to gear than SET. Even though SET is ultimately a better fit for this squad (and can move on to your Revan/Malak squad later so the gear will never be wasted), in the short run there is a reason to pick Maul over SET. Only you can decide how important it is to get the perfect pick vs. how important it is to gear your pick up fast. If fast gearing is important, Maul would likely move up to #1.