Youmuus ghostblade.

Last Whisper for AS and ArPen (towers have lots of armor), after that I'd just stack dmg items.

After midgame, if you've been doing an AD build. Just pop E+R and you should take down a tower in no time.

Well if you have one AS item (Last Whisper etc) and you pop Highlander and Wuju Style that tower's going down. Of course you need respectable AD as well.

In general, you want a balance of attack damage and attack speed, and possibly armor penetration. Avoid critical chance since you can't crit towers.

The more attack speed you have, the more damage helps, and vice versa, so don't go super crazy on either one... but remember attack speed has a cap of 2.5 (and your ultimate speeds that up), while attack damage has no cap.