While there are numerous ways you can build your Dark Souls 3 characters based on which weapon you prefer using or what type of play style you enjoy, you could choose one of the following endgame builds as a baseline to customize your character around.
Classes: Knight, Warrior, Mercenary
This is a balanced build where you equip a strength scaled longsword or greatsword. If your weapon has dexterity scaling, you should level dexterity instead of strength. If your weapon has good scaling with both strength and dexterity, level those two stats equally. As always keep in mind to keep your equip load below 70%. The Fallen Knight Armor set works well with this build and throughout the game. The Dark Sword is an excellent choice for this path but most other weapons work well too. The Dark Sword can be acquired from the Dark Wraiths before the Abyss Watchers boss fight area.
A standard pyromancer end game build, where you equip a longsword or mace and max out your Pyromancy Flame. Chaos Fire Orb does a lot of damage so it’s a must have. Other spells can be chosen according to your playstyle. A healing miracle is handy to have attuned as well, so equip a Saint’s Talisman or Cleric’s Sacred Chime for healing. If you want to buff your weapons, you want Carthus Flame Arc or Dark Blade.
This Sorcerer build will allow you to have 5 slots of attunement. Sorcerer’s Staff is great throughout the game, but Crystal Sage’s Staff is an amazing end game staff. Crystal Soul Spear will be your main nuke damage, while Soul Greatsword offers excellent aoe coverage. Other useful spells include Flash Sweat for fire resistance, Power Within to buff damage and Tears of Denial for a second wind in a boss fight. If you need more survivability in boss fights, you could opt for Magic Shield too.