Nasus Has been my Favorite champ for a long time, and fits into that grey area of tanks that still hit like MAC trucks, with only one or two real damaging items. Through the same procedure as my pantheon build, I have perfected Nasus for my playstyle. This build will not go in depth, it is just here as an additional Nasus buildstyle reference, not as a 'one stop shop.'

Pros
-Incredibly Beefy, nearly impossible to take down with ult up.
-Q hits like a MAC truck when it is properly farmed
-Spiritfire DESTROYS an enemy team at max level, and makes tanks less of a problem, turns squishies into warm butter.
-Wither is AMAZING for ganking, denying a gank, minimalizing a DPS' damage, or chasing that squishy into tower range.
-A tower-taking MACHINE

Cons
-Vulernable with ult down (not that much though)
-Q is worthless if not farmed, so it CAN be easy to shut Nasus Down with a smart enemy team.
-Late game is only good in the Q department, it is when the enemy team catches up to you and you are not that formidable 1v3 winner that you were midgame.
-People on your team tend to let you do things yourself, ie: leave you REAL quick in a teamfight.

Nasus is focused around his Q, so last hit with the q. Solotop if you have a jungle, just farm, farm, farm. Ignore minor skirmishes until your q is beyond 290 additional damage. after your q has been farmed up sufficiently, take your tower, switch lane, push them out, take a tower, and repeat. if you are not sure where you should be, be farming in you lane. your team success is based almost entirely on how farmed your Q is (on a mirror match anyway)

I have always found these to be purely subjective to what your playstyle is going to be. Though i recomend that you get one mobility summoner spell and one offensive, usually to pick up the slack in one area on the enemy team.

Getting the Sunfire Cape is essential on Nasus for maximum AoE damage, for it synergizes well with your ult, gives you armor AND health. Sunfire Cape takes a back seat to Force of Nature against a heavy AP team though, but these two items are usually taken at the same time.
A thornmail can replace the Force of Nature if they are almost all AD, or the AD is overfed, in which case, let the MR from Merc Treads and Aegis be enough for the underfed AP's.
A bloodthirster is for when you are having a good game, and you just want to finish it faster, 2 shot most carries with your q, or 4-5shot beefy enemies with your q. The lifesteal in combination with your passive, you will live for a sufficiently longer time. Unfortunately, we seldom have great games, in which case a GA makes you all the more fearsome and gives you some good MR and Armor; it also makes enemies want to focus you down, filling your tank role better, especially if you are eating them alive with a well farmed q.
Aegis of the Legion is 100% require of a tank of any kind. Great aura, good MR and armor, plus health, CAN'T say no to that.
Sheen is Nasus' item. No 2 ways about it. You WILL get Sheen first. FIRST, as in BEFORE BOOTS. I tend not to build it into Trinity Force until after my two main tank items (Sunfire Cape and Force of Nature)

I have never had a good game building pure health. I have seldom lost with a sunfire cape on the other hand. I prefer damage on ALL nearby enemies and armor to more health period.
God knows i love Atma's Impaler, and if they have a heavier AD team and you still need damage, it is PERFECT for this, but I tend not to build it on my Nasus. You build it if you see fit.

I am not a huge fan of Banshee's veil. It does give good health, good mana, good MR, and an AWESOME passive, but the passive pops, so it will only work on the first hit of the fight, which better fits the carry role of only taking one or two hits, but Force of Nature gives AMAZING MR, more movement speed (which God knows tanks could use), also, the additional Dr. Mundo Passive is nice.