I would say Great Forge, because the shields are a huge upgrade to what you can make otherwise.
The sets is also pretty nice, as you only have to order full sets to be made, but when you have a good manager, it doesn't really makes that much of a difference.
And when you know a bit about dwarf fortress, you should also know that shields tend to be extremely usefull, especially at high levels, with better base shields making your army even harder to kill.
Technically speaking, two handed weapons have a pretty insane power, but knowing that you can't have a shield, it makes your warriors much less durable, and you really need high grade armour (both material and quality), as well as a high level in armour skill to be able to compensate.
The Toxicology lab becomes really nice when you can safely farm the plants from the third cavern, as they all produce pretty potent poisons, but as said above, you will need a pretty high degree of micro management to be able to put poison on the weapons you actually want to poison.
It does stack with runes, so if you are going for an "ultimate" weapon, then poisons is a step to achieve that, but realistically speaking, there are very few things that you actually need those godly weapons for.