Q: Use it for harrass (early game). Use it to last hit minions like a boss. Use it to finish nearly dead runners. Unless doing so ensures the death of an enemy or two, DO NOT waste your W to get more damage from this ability. Max 1st for early harrass and farm (if in lane).
W: It is a flash that can also damage/cc/kill chasers/runners, or just get you free assists. It has a pretty short CD if you think of it like that. It also acts as a free AD version of Rab's Deathcap when maxed. DO NOT use W if you think there is any chance you may be ganked within 20 seconds. Max last (cause +25% bonus AD is useless if you only have 50 bonus AD).
E: Great for mid-late game farming. The slow is not much, but can be spammed on multiple targets. For maximum chasing ability, do not use Q (it stalls Zed for a moment).
R: Free short version of Zhonya's. Also a gap closer. Also leaves one of those cool flashes (look at Q for referance). Makes a better gap closer than your W. For a bit of fun, use it on an enemy with low hp, then burst and auto attack a healthier target as your ulted target runs away.
DO NOT use R when your target clearly has an unused escape mechanic. Force your target to use their escape mechanic first THEN use ult. Also DO NOT use your ult on targets that are clearly killable without your ult.
Zed is bad as a tanky dps. As a tanky dps he has no damage and only has one cc and it is bad. I firmly believe Zed is best built as a carry, and should only be used against teams with at least three squishies.
With full armor pen runes/masteries+IE+SotD (active)+BC+LW+BT+ult. Zed is capable of a four second burst damage total of 3500+ against opponents with 150 armor.
Inb4 downvoters/zedhaters/zedexperts/tolazytoreadwholeposters/lolkingnazis/gramarnazis. Please at least try these ideas a few times.
I do not flame often, but when I do, I flame irrationally. Rage responsibly my friends.