This is because it kills marines and
MCs very well, and marines and
MCs are common. It's not the only viable option, but it's definitely a good one.
- I'm worried about my lack of turn 1 AT (the lightning, stormies and marbo being my only AT)
This is definitely a problem. Marbo isn't anti-tank because a single
STR 8 shot is not even close to consistent enough, and the Lightning isn't anti-tank* because it only has a single
TL LC to threaten tanks with. Your tanks need to be capable of dealing with tanks, and you need to replace the Valkyrie with a Vendetta.
*Or anti-anything, really. The Lightning is garbage, proxy it as an Avenger.
What would be an ideal combination for an all rounder army?
Either the Demolisher or Vanquisher. You need more anti-tank, so those are your two options. The Demolisher is shorter-ranged but can handle more roles, while the Vanquisher gives you range but is a single-role specialist. Either one will work, so it depends on your personal preferences.