When Space Marine Legions are in a siege, their core troops are the Legion Breacher Siege Squad. Not only are the the core troops of a siege, they are also vanguard assault force in the worst conditions of war. Some Space Marine Legions have a strong preference for these kinds of warfare, specifically the Imperial Fists, Iron Warriors and World Eaters.
The Legion Breacher Siege Squad is literally designed for Siege Warfare and Zone Mortalis. In all respects, this squad would be my go to choice for any Zone Mortalis game (it fits theme and they count as Void Hardened). Otherwise, you can treat them exactly like a Legion Tactical Squad. Hell, they are almost the same (emphasis on the word almost).
Like most units in the Army List, the extra 10 Space Marines are extremely cheap (10 points per model). However the initial 10 are quite expensive. As I stated before, you can treat them exactly as you would a Legion Tactical Squad. They are more survivable, with a 5++ in Close Combat, and 6++ in range (in my experience, a 6++ Black Templar proved to be extremely survivable). In addition, they have the option of taking Special Weapons (a maximum of 4 in a 20 man squad).
Personally, I still say the best troop choice is a Legion Tactical Squad. And like the Legion Assault Squad, this squad might be best left to thematic (or Zone Mortalis) armies.
Melta Bombs - Nice, but you have to take it for the whole squad so it becomes too expensive. Would've worked well if you could take it for a few models.
Volkite Charger - A pretty cool weapon. It's like a stronger but short ranged Storm Bolter.
Flamer - A fun weapon, but doesn't work too well on Foot Sloggers, leave it behind.
Meltagun - We all know what this is, but you can leave the anti-tanking for more specialised squads. Ustilise the mass amount of Bolters you'd have in this squad.
Graviton Gun - This thing, I really like. The opponent must roll against their Strength or suffer a wound.
Lascutter - Personally, this is a no go. A "no go " in caps. Close Combat only, you strike at I1 AND at WS1. I really don't see the point.
Nuncio-Vox - Quite useful if you have a host of Barrage type weapons or perhaps Deep Striking units.
Legion Vexilla - As usual, take it, it'll stop them from running, or at least make them less likely to run.
One Breaching Charge - It's like a Melta Bomb, but more devastating and a chance to kill your own men. One use only too. I guess it's up to personal preference on this one. Personally, I'd avoid it. I don't like things that can potentially kill myself.
Melta Bombs - Since it's better to take Melta Bombs for a few models, the Sergeant will suffice.
Artificer Armour - With the new Challengers, this might actually prove to be a really good choice.
Combi-Weapon - I've never liked Combi-Weapons, so it's totally up to you. If you think a one-shot weapon is good, then go for it.
Plasma Pistol - Too expensive!!
Power Weapon - Power Sword or Power Maul, your choice. Power Axe would be nice, but I'd rather take the Power Fist.
Power Fist - If you take this, you don't need Melta Bombs. Coupled with Artificer Armour, you can prove to be quite deadly in Challenges.
Single Lightning Claw - I've always believed Lightning Claws should be taken as a Pair or with a Storm Shield perhaps (unless you're in Terminator Armour). So personally, I say no to this option.
Thunder Hammer - I love Thunder Hammers, but a Power Fist would be better for you.
Suggested Option 1: 425 Points (Apothecary not Counted)
20 Legion Breacher Siege Space Marines with 4 Graviton Guns and Legion Vexilla. Sergeant has Artificer Armour, a Power Sword and Melta Bombs.
Apothecary Attachment (More on this at a Later Date)
Treat this like you would your Tactical Squad. You can hold objectives extremely well.Two squads of these can form a strong backbone for your army. Take a Nuncio-Vox if you have Barrage type weapons or have Deep Striking Units. The Apothecary can make this squad extremely survivable.
Note: The Veteran Upgrade rule that are listed as having; is from previous play-testing rules that was overlooked by the editor (Forge World answered this on Facebook). So you can ignore this.