I prefer Teleport for solo top, but you can take exhaust as well. If you are a very offensive player or if the enemy team are very tanky you can also take ignite

My runes for nasus are equal as them for an ad-carry

My marks : Armor penetration
Seals : Armor
Glyphs : Magic resist
Quintessence : Attack damage

I start with life regen and a health pot.
then i go for Philosopher´s Stone
i build boots and sheen
then i complete my mercury´s treads.
for a little bit armor and magic resist i buy a chain west and a negatron cloak
i build glacial shroud and Force of nature
i complete my frozen heart and go for warmogs armor
for the endgame you´ll need much resistance for this the only item = guardian angel