Mid diamond Nasus main last season here.
Vs Physical Damage:
9x Armor Pen or 9x Hybrid Pen Reds -don't run AD unless you really suck at lasthitting, Q's bonus damage has an AD ratio of zero but scales with armor pen. Take Hybrid Pen against ranged champs with no MR scaling; 5 Pen on 30 MR is worth considering that you're also putting an early 2nd point in E in those matchups.
9x Armor Yellows
9x CDR Blues for 7.5% CDR at level 1, a CDR quint puts you at 10%. Alternatively, you could run scaling MR here and go for FH + Sheen + Kindlegem for the cap, but that's really risky as it'll take a long time to get all those components
1x CDR, 2x MS Quints (One CDR quint to hit 10% at level 1; MS is the strongest scaling stat you can get in quints. If you really want to optimize for early game, run 2x Armor Quints instead of AD. Again, AD runes on Nasus are a complete crutch as his kit has a combined AD ratio of...zero, which means you're only getting them for the lasthitting. Get better at lasthitting and ditch the AD.)
Build Path 1: Iceborn + Kindlegem: With Iceborn at 20% CDR now I feel like this start is more often the go-to than the FH + Sheen combo, especially considering the fact that you're getting the CDR faster (FH doesn't give its full CDR until you complete it, unlike Iceborn.)
Build Path 2: FH + Sheen: Of course this is still viable. However, preseason hit Frozen Heart pretty hard; nowadays you're really paying a premium for that Cold Steel, which means that you should only go for this in matchups where the attack speed slow makes a huge difference, e.g. Trynd.
Build Path 3: FH + Sheen + Kindlegem: If you're running 0% CDR in runes, go for this to cap out. I don't recommend this in most cases, as it takes far too long to get the 40% and forces you into going FH in lanes where it might not be fantastic.
Vs Magic Damage:
9x Armor Pen or 9x Hybrid Pen Reds -again, strongly recommend Hybrid Pen versus mages with zero MR scaling, as it adds massively to your E and ult damage. Versus Teemo it's a must-have, as you're maxing E.
9x Scaling Health Yellows since mage autos don't do enough to justify Armor here.
9x MR Blues -you can mix in some scaling MR too, depending on when the lane opponent spikes. Don't try running CDR here, it isn't worthwhile.
3x MS Quints
Build Path: Kindle + Sheen -> Visage. Honestly the no-brainer choice here; Visage is fantastic, as it both synergizes with Nasus's passive and also conveniently builds out of the only CDR item that provides a relevant defensive stat in a mage lane. Now you can go greedy and sit on Sheen -> Glacial -> FH for the cap, but again I don't recommend this unless the lane is trivially easy post-9, i.e. AP Shaco. Usually you're better off getting Iceborn and either asking for blues or getting Caulfield's for the last 10%.