This guide for Nasus is currently under development. I'm trying my best and I'll release the half done guide soon.

Stay tuned, Tomica1996.

Hi, My name is Tomica and I'm a Nasus lover, no lifer and expert.
This is my first guide ever and I'm gonna try my best to show you how I play with Nasus and how to dominate lanes with him.
Sorry but I'm not good at BBCode's so expect alot of texts and reading. (I'm gonna fix it trough time)

Pro's

Durable And Sustainable
High Damage If Played Properly
AD-Carry Counter Due To Wither
Great 1vs1 Due To High Damage, Slow And Armor Reducion
High Base Speed Whitch Makes Him A Great Hunter
Strong Pusher If Siphoning Strike Is Well Stacked

Con's

No Dash Escape
Only 1 Single Target CC Wither
Lacks Damage w/o Farm On Siphoning Strike
Kiteable In Team Fights

When I play Nasus on top I have a feeling that I have only few options that can counter me.
Those options are:

Ranged ex. Teemo
Sustainable ex. Vladimir
Relaing On Skills (So it can not be countered by Wither (for instance Rumble or Riven)
Fast And With Dashing Abilityes (like Renekton's Slice and Dice or Garen's Decisive Strike)

Under champions I don't like to meet in top I puted;

Vladimir - Getting this guy as a lane matchup is not so fun. He will spam his Transfusion on you. Also, if your makeing "trades" with him; stop! Cuz he can heal much faster with Transfusion and still deal dmg to you. Try to farm as much as you can and ask for ganks. I palyed against him only one time and I won due to no ganks recived from our jungler. Don't know if that Vladimir was so bad or what..
Teemo - What to say, Blinding Dart, miss, miss, miss, miss.. Yeah, probably the most annoying, iritating and imposible matchups you will get. His blind with Blinding Dart will make your damage output minimal due to misses with your Siphoning Strike. His Toxic Shot will make your farming hard due to DOT. I got him a couple of times and I just waited for ganks and then easly killed him.
Pantheon - The ONLY. And I mean ONLY matchup I didn't win against EVER. His Spear Shot that costs no mana, has extremly high damage for the CD it has. His Aegis Protection will just eat your Siphoning Strike if you don't pay attention to it. I had him many times and everytime he gets fed. Don't know how and why, but I hate him the most..

Also, there are champions I just enjoy getting as a matchup. Those are the champions that have no range skills and that relay on AA's. Those champions give you space to farm freely and with no stress.

Those champions should be something like this:

Melee
Relaying on AA's (For example; Vi that has to AA so she can get her 3rd AA Denting Blows)
No crowd control removers or long range dashes (ex. Zed or Wukong)
Mana Hungry ( Nasus just denies those mana hungry champions due to his massive sustain with Soul Eater)

I know you're all surprised that I putted in that 2% critical rune, but belive me, it's the best rune I had ever used. It has helped me so many times. Many times I got a lvl 1 critical and got 1st blood.

For marks I used Mark of Attack Damage because it makes farming minions and stacking Siphoning Strike in early game easier and faster.

For seals I used Seal of Armor and for glyph Greater Glyph of Scaling Magic Resist for later game MR or Greater Glyph of Magic Resist for early game MR. These runes should make you tanky enough if you play it well. You should survive ganks due to Wither and Ghost mobility.

For quints I took, as you know, Greater Quintessence of Critical Chance and 2 Greater Quintessence of Attack Damage just to make your harass more painful.

I took 9/21/0 masteries so you can be tanky but still make quite some damage.

At Offense I took Summoner's Wrath mostly to make your Ghost more usable at tower diveing and hunting or escaping danger.
If you wonder why I maybe didn't choose Sorcery it's beacuse trough the game, the build will give you max CD (40%), so Fury may help more trough the game progress.
At Defense I went for a more health based masteries because the items that you'll buy will give you enough MR and Armor so masteries for Mr and Armor wouldn't be so useful.

At the first three levels try to get all three spells just in case you get dived, ganked or the other way around.

Against AP champs as Vladimir max out your Siphoning Strike to make your farming faster, sustain better and damage more furious. For the second max spell there are 2 choices; Wither in case your enemy isn't buying armor so your ganks can get faster and escaping easier and Spirit Fire in case your enemy is building armor but your Wither, in that case, will be almost useless, but still will come in handy. When you max out Spirit Fire you become more aggressive so on you can take multiple champs easier because you shred their armor and you deal massive damage and have massive sustain.

Against AD champs as Wukong you should max out Wither if he's aggressive (and most of them are) so you should buy Glacial Shroud and Kindlegem to make your stack on Siphoning Strike stacking like usual. For the second max I take Siphoning Strike so I could farm and make kills easier. If you want you can max Spirit Fire 2nd but I would rather go for Siphoning Strike.

Still in progress of making the guide.