He's very flexible.
Starting (Choose 1-2)
Poor Man's Shield: Passive early game
Orb of Venom: Aggressive early game and for Skadi rushes
Bottle: If you're mid or your mid doesn't need runes
Magic Wand: For spell spammers
Core (Choose 2-3, including treads)
Treads: Almost always. Keep it on strength for fighting unless you're good enough to switch modes mid fight, the HP is invaluable for Slark.
Yasha: More speed and chase
Drums of Endurance: More HP and chase
Ultimate Orb: If you're really badass
Mask of Madness: If you're really really badass
Midas: If you're me
Midgame (Choose 1-2)
Diffusal: Good balance of chase and damage
Basher: If you're fighting other harder carries often
S&Y: If you're doing a lot of chasing against runners and you need a little more HP
Orchid: If you need to shut down blinkers/casters like QoP, AM, or OD
BKB: Self explanetory
Skadi: Huehuehue (only if you're rushing it from the Core)
Lategame
Abyssal Blade: Progress from Basher
Heart: If you need to initiate often
Butterfly: For fighting other late game hard carries, not as good as Abyssal generally
Monkey King Bar: Countering Butterflies
Divine Rapier: Huehuehuehuehuehuehuehue
Other items that are good on him but I rarely use
Mjolnir
Assault Cuirass
Armlet
Linken's Sphere
Scythe
"But Haly, this is every item in the fucking game."
That's right, Slark can make any item work, he's just that good.