Tools and weapons – taking an extra sword and pick might be a life-saver, or at least very helpful, after prolonged fighting or mining. Take a strong sword, just don’t drop it near lava!
Torches – here, extra is unnecessary. Fortress corridors can be dark, but lava and glow stone are everywhere.
Food – take extra to avoid starvation, depending on how much time you’re planning to spend in the nether. As always, factor in fighting, mining etc.
Armor and sword – consider taking your best into this dangerous hell-scape (still keeping far from the lava!)
Bow – very effective against ghasts, and will deflect fire charges. With arrows to spare, you can shoot other foes at a safe distance too, like the blaze or poisonous wither skeleton.
Flint and steel – to relight the portal, should it get broken by a ghasts fire charge (alternatively, lure a ghast into shooting you and relighting a broken portal with another charge).
Resources – take more cobble. Only build with this, as its safe from both fire and charges. If you wish to build around the portal for protection, use iron bars for glass. Dirt is still good for any makeshift bridges or pillars.
Fire protection – this enchantment or the potions of fire resistance will do you justice, for added defence from fire and mobs. Not the lava though!
Other – Ender pearls are relatively hard to get, but may save your life in case you accidentally fall into lava. A water bucket, map, bed or clock will be of absolutely no use in the nether.
Part of my concise inventory series: https://premiumminecraft.com/survival-inventory-layout-and-space-management/