i'm saying monolith simply because when used right it can make the necrons utterly invincible...
it has great armour & protection in the form of the 'living metal' special rule. only S9+ ordinance and the railgun can reliably hurt it...
finally it's a skimmer that isn't destroyed when it's immobilised.
it's one of the few vehicles that functions like a 'taxi' with the added bonus of teleporting, even 'necron' units locked in assault can be saved via the teleport...
as for weapons, it has superb in-close defense weapons which fire at nearby enemies upto 6 times...
the cruelest thing of all though is the teleport's effect with the 'we'll be back' rules; units that teleport can re-roll any failed WBB rolls!
and you can combine the effects of a res orb too. when facing a pair of these behemoths and a pile of warriors, it become very difficult to keep any of them down while the 'liths are active...
cheers!