Nasus is my go-to champion and even though many people start whining in the lobby about me taking exhaust and clarity, they start to notice why I took these summoner spells instead of other during the game. I don't think my build is the one and only, but this build works in 99% of the games I've played.

I am able to do a ton of damage with this build and also I have a lot of survivalability to escape ganks or help teammates get out of trouble, and my Clarity spells is often used to surprise greedy players when they tower dive me.

I'll explain most of the build later on, I just hope it helps other Nasus fanatics :)

I chose these runes for Nasus after trying several others but my game improved dramaticly when I started to use Greater Mark of Alacity, Greater Seal of Malice and Greater Glyph of Warding. For Quintessence I chose armor and movement speed. I like the extra movement speed because Nasus is a old dog who apperantly doesn't have the stamina to sprint at a fast pace so the extra movement speed often gets me the killing blow of a running away target or lets me escape easily.
Not to mention that it helps getting to teamfights when they are about to start.
However, I've not tried all runes for Nasus yet so I don't know what else to recommend.

As for the skill sequence, I hear that many, if not all Nasus players max Siphon Strike first. However, I think that maxing Spirit Fire first does more damage since you need to farm Siphon Strike first to get a decent damage out put not to mention that E is your only "ranged" ability which often can be used to last hit a fleeing champion. I do realize the drawback for maxing E first, but thats what clarity is for. If I play solo lane I tend to use clarity during middle of lvl 3 so the cd is done when I turn lvl 6, but try using clarity so that your oppenent does not notice. That way you can surprise him. Example: you turn lvl 6 without having gone to base, you're probably out of mana by now and low on health, but you have ulti ready. Your oppenent on the other hand has most likely gone to base once or he is too greedy. Chances are good that he will tower dive it comes to an engagement and you flee to the turret. Now you use Clarity and ulti, slow him with your W and then throw a Spirit Fire and follow him with Siphon Strike and get the kill. Of course you can also just do it in mid-lane if you know you won't get ganked. This often get's me first blood.

For Wither, I only get lvl 1 because I have exhaust and with the double slow, his escape is almost impossible unless he has flash.

It's very importand nevertheless to last hit minions with siphon strike!!!

As I've said earlier, I highly recommend Clarity + Exhaust. However another alternative is Ghost + Clarity. Though I prefer Exhaust.
Clarity is in my opinion a must skill on Nasus, not only because Spirit Fire eats your mana but also because it can turn around 2v2's or team fights.

For masteries I go a combo of dps and survival since many people expect Nasus to be the Tank or Off-Tank. Even for 1v1's Nasus still packs enough punch with this mastery build to beat pretty much anyone.

After a lot of games I've started with Doran's Shield, I think my entry items are better. I skip Doran's shield while I start building for my Ninja Tabi. The ward is replaceable with another health potion but wards are so usefull, especially if you are going solo top vs solo top. It helps both you not getting ganked, but also prevents your jungler from running into an attempted gank from the enemy team.

The only items I sometimes swap is the Warmog's Armor and buy Stark's Fervor BEFORE I buy Randuins.

And if it's an AD/AP heavy team I change Randuins for either Force of Nature or Thornmail, depenting on the situation.

I hope people give this guide a shot, I'm not aiming to write the best guide for Nasus.
I just wanted to share my opinion and idea of how I play him.

I'm sorry if the guide is not presenting well but I have to admit that this is the first time I've tried writing one. Please comment to help me improve it or add your ideas. Thank you!