Nasus is a very good tanky-DPS champion (also called a bruiser and a fighter,) and he's an excellent candidate for soloing top. Indeed, you should more or less ALWAYS solo top with Nasus, which I'll explain later. If you build him as a tanky-DPS carry, then Nasus should be chunking off massive blocks of health from the enemy by late game.
Awesome Stuff about Nasus and Not-Awesome Stuff
High damage output, both per second and burst.
Stupidly high sustain from lifesteal by end game.
Pretty tanky. 3k health is pretty good for a non-tank.
If he doesn't get farmed on his Siphoning Strike, he turns into a puppy.
He's pretty kite-able if you know how to do it.
Like all DPS champs, a good amount of CC will destroy him.
There are two primary ways to build Nasus: leaning more towards his tanking abilities or leaning more towards his AD-carry potential. This build capitalises heavily on his DPS potential.
I start off with basic boots and 3 Health Potions. The boots give me a nice early game speed boost for chasing down a First Blood or maybe getting away from giving one. Combined with the Movespeed Quintessences, you'll be running pretty fast in the first few minutes.
The three Health Pots might be called overkill sustain by some (given Nasus' natural lifesteal and his Siphoning Strike,) but I take them anyway--just in case. Besides, they don't set you back that far if you lane and farm well.
The next item I get is Berserker's Greaves. This is just for the attack speed to make laning and last-hitting easier to accomplish. If the enemy team is CC-heavy, though, be sure to grab Mercury's Treads. Choosing Merc's over Zerks will affect the end game build a little, though, so keep that in mind.
Now I go for a Zeal, boosting my attack speed further and giving me some critical chance to boot. I follow up immediately with a Phantom Dancer. It might seem strange to be getting a PD before building up any AD, but I find that at this point in the game the combined effects of the Movespeed boost, AS boost, and Crit chance boost are more useful.
Now we get ourselves some damage with a B.F. Sword. Fairly simple and self-explanatory. However, instead of rushing an Infinity Edge, I opt for a Vampiric Scepter instead. This allows me to take even greater advantage of Nasus' passive lifesteal. Be careful not to forget what you're building and accidentally rush a Bloodthirster, though.
After the Vamp Scepter comes the Infinity Edge. If you've ever played an DPS champ, you know the power of PD + IE. Long story short, you now hurt that much more.
Now that our basic damage output groundwork is laid down, it's time to start getting a little beefier. I take a Giant's Belt and follow up with a Frozen Mallet. This gives me a good health boost, and Frozen Mallet's slow combined with Nasus' Wither almost ensures that your target won't be going anywhere.
After that's done, we go back and build more damage with a Bloodthirster. Simple enough.
A lot of games would be finished by now, but in case you're in a long, dragged-out game, these are the optional end-game items. First, grab a Trinity Force. Hands down, this is just one of the most broken items in the League. It can't do everything, but for Nasus (especially at this point in the game,) it's freakin' awesome.
If you bought Mercury's Treads earlier, ignore this last bit. If you bought Berserker's Greaves, though, then you may want to consider selling them and buying another Phantom Dancer. Just a thought.
As mentioned earlier, Mercury's Treads can be a good alternative to Berserker's Greaves if the enemy team is heavy on CC.
The Trinity Force at the end is just an extra "**** you" to the enemy team, so if you want to have more survivability as opposed to killing power, consider buying a tank-ish item. Randiun's Omen, Force of Nature, Warmog's Armour, and Guardian Angel are all good candidates to replace the Trinity.
Whichever way you play Nasus, people will almost always tell you to max Siphoning Strike first. I am no different. Siphoning Strike is Nasus' primary farming tool early game, and one of his primary damage tools late game. GET IT FIRST.
Siphoning Strike's farm-it-and-it-gets-buffer clause is why you should always solo top Nasus. If you have someone else sitting around taking your farm, your Siphoning Strike won't be getting the damage boost it needs and deserves. Whenever possible, use Siphoning Strike to last hit. If it's not up, though, don't be afraid to just last hit normally.
My next highest priority is Wither. Wither is one of the most powerful single-target slows in League, and it has pretty good cast range as well. It's a valuable chase/escape tool, so max it second.
Then there's Spirit Fire. While you CAN lay this down in front of your tower to prevent pushing or you CAN lay it down in lane to prevent enemy farming, it's not worth it. The mana costs are too high and hungry early game for it to be of much use, and its late game damage just kind of...falls off. Unless you build AP Nasus, but I've seen that, and frankly, it sucks nuts. Putting down Spirit Fire on a minion wave also inhibits Siphoning Strike farming. Just a little. All in all, Spirit Fire isn't that great. Don't get it until Level 13 and save your skill points for the two more important skills.
Like all ultimates, level Nasus' Fury of the Sands at levels 6, 11, and 16. Fury of the Sands is especially powerful in teamfights because of all dat AoE. Use it somewhere near the middle of the team fight (both in terms of position and time.) While you CAN pop FotS to initiate, I don't recommend it. It's better off as a lifesaver/mopper-up. You can also use FotS as an instant health pot if you get ganked in lane or things somehow go wrong. And you can tower dive with it. The possibilities are endless. It doesn't make you invincible, though, so exercise caution.
He Was Just a Humble Farmer
Like I said in the Skills section, farm with Siphoning strike whenever you can.
Siphoning strike gains 3 damage permanently every time you successfully kill something with it. Think about it. 3 damage x ~250 CS = 750 bonus damage for Siphoning Strike. Buff ****.
Again, making sure you solo top with Nasus is a very good idea. All the minions shall be yours. ALL OF THE MINIONS. :allofthethings:
Nasus is pretty resistant to in-lane harass, so don't be afraid to extend just a little bit if it means you'll get another stack from the caster minion in the back. Still, be careful. After the laning phase, you can run around in the jungle whenever you feel like it to grab more stacks. Make sure your jungler/the rest of your team is okay with it, though. Especially if you're going to be grabbing red and/or blue buff. Pissed off teammates are the downfall of any champion, no matter how good they are.
I'm ASSUMING that your team comp is good. If it's not, dodge queue, get a premade 5 (or 3) and get a good team comp. Good team comps are good. Game plans win games. Ahem.
Assuming that your team comp is nice, this Nasus build will fit in quite nicely. Nasus solo top, have a support-carry bot, and have AP carry mid. And then there's your jungler. Pray to God you have a good jungler, because Nasus (like any solo top) will partially depend on good ganks from the jungler to both farm and push effectively.
Since this build makes use of Summoner Teleport, it might be wise to save it instead of using just to get back into lane faster. While this may allow you to farm more/prevent enemy push, it's just as good to be able to hop into a gank gone wrong across the map and save the day. Always be thinking about your teammates.
At the beginning of teamfights, lay down a Spirit Fire in the area you think the fight will be in. The damage over time isn't THAT great, but the Armour debuff helps a lot, especially for your AD carries (including yourself!)
Also in the spirit of the save-your-Teleport-to-save-lives tactic, consider saving your Wither to peel the baddies off of a fleeing teammate, instead of popping it to secure yourself a kill.
Nasus is pretty tanky, but again ASSUMING your team comp is nice, he should only be doing SOME of the tanking. This build's main focus is his damage potential.