As you may have noticed, most of my builds are solo top champions. Thsi one is no exception. Nasus is a great solo top champion due to his reliance on the farming and last hits he needs with his Q, Siphoning Strike. If farmed correctly during a solo top laning process, this move becomes extremely powerful for late game team fights, and with the added item build, an hit criticals and use proc damage. Not tank Nasus.
Flat armor penetration marks help any Attack Damage based champion usually only up to about early-mid game. Although this will help deal damage to the enemy with Q when jungle comes to gank. Attack damage seals are kind of a filler, but they do help farm the Q early as the rest of the runes are for the on hit critical chance of Siphoning Strike. This way, critical chance is improved early game so when you land critical strikes, they hurt, sort of a bruiser style Gangplank.
The masteries I use for solo top Nasus focus on cool down for faster Q farming, but also all the damage possible from the Offense tree to make Siphoning Strike the primary damage for Nasus' build. It focuses on damage, so the runes will still be put to use lacking as much damage by using critical chance. This way early criticals will deal lots of damage for early game ganks and so forth.
Take the skills in the order listed. This will focus on leveling Q for the build and getting wither at level 3 will help the jungler with ganks. This will focus on scaling Q early and fast to hit harder and leave more wounds on the enemy, forcing them to recall often.
Exhaust is a good summoner spell for melee champions. That being said, in addition to wither, nasus will be one of the best slowing Crowd controls in the game. This will not only help you early with ganks, and lane harass, but wil also help your team in team fights, especially shutting down the enemy AD carry. Flash is good to take because along with that and Wither, Nasus can escape better.
These items, give Nasus the early game solo top leverage he needs to farm his Q, Siphoning Strike, but also builds him into a powerful team fight critical striker. With the critical chance and added damage, in addition to a properly farmed Q, you can deal TONS of damage. When The Bloodthirster is complete, the Doran's shield is sold for the Zeke's Herald. If the enemy is building armor however, it may prevent your Q from doing max potential damage. In this case, buy a Last Whisper instead of Zeke's Herald. sprit Visage is another option as substitution for Zeke's Herald, adding cooldown and bonus healing. Other than that the build is focused around the on-hit effect and critical of Siphoning Strike. Life steal is another important trait to have on a solo top champion. As Nasus already has life steal for a passive, why not make it better and get lots of life steal items. Hextech Gunblade will get lots of health back when you use Nasus' Ultimate in team fights, in addition to The Bloodthirster's addition to Q damage and lifesteal.
Ask your team's jungler for blue buff, as it is important you have it for laning at least 1 time. Having blue buff will aid in your harass of the enemy. Withering them will annoy them, and may deter them from attacking you or harassing back. Wither costs more mana than other abilities, so i recommend using it sparingly, or get blue buff as much as possible so you wont have to worry much about mana costs.
It is #1 priority and of the utmost importance to farm the Q, Siphoning Strike properly. If you dont take up the early game advantage and force the enemy out of lane as much as possible, this could hinder your farm. Every kill you attain with the farm, gives it +3 permanent damage. And every kill makes you stronger and stronger.
Pros / Cons
1. End game damage from Q is very high, especially with criticals.
2. Life steal will get you further in team fights, sustain better in lane, and allow you to do more damage.
3. Critical Strikes do so much damage with Q.
1. Not a tanky build
2. Not even off-tank