My favorite build...
Starting Items:

Core Items:

Offense:

Defense:

So I'm sure right now you're asking, "Why is
core? Where is the
? Who let the dogs out?"
's ult does 3%/4%/5% + 1% per 100 AP capped at 240 magic damage every second for 15 seconds. Your ult continues to damage through the
active. If you combine that with Rylai's slow and 300/450/600 bonus health plus your resistances, you can stick to your target or bait with your active effect to get the maximum burn in teamfights. A full Nasus ult does a maximum of 45%/60%/75% + 15% per 100 AP capped at 3600 magic damage, and this is what makes AP Doge secret OP. (This is the same reason why you want
)
Nasus has a historically abysmal early game, only able to safely CS when his lifesteal, potions, or items sustain him through enemy damage. Building AP allows you to safely CS while shoving with no personal risk. Therefore, I recommend you don't even put a point in Q until forced to, getting more slow from your W instead. If you are in range to autoattack, you should be ulting or running away, so buying a sheen can almost bait you into feeling like you need to get close to a minion wave you could easily evaporate with an E.
I don't know who let the dogs out, try asking the Baja Men.