Best Diana Builds
This is how I build my main, 
Easy lane, lots of Squishies on their team:
Harder Lane, Against other Divers or CC Bursts:
When they have multiple tanks:

This is how I build my main, 
Easy lane, lots of Squishies on their team:
Harder Lane, Against other Divers or CC Bursts:
When they have multiple tanks:

I jungle
but it depends on the champions that are picked, otherwise I play
.
I am not a jungle main, but I picked her up in the jungle because my cousin is a platinum diana player and he recommended her when I was having a hard time finding a jungler that I liked. I tried a bunch and all of them I didn't really like too much (
. He just pointed out that I tended to farm more than the average jungler and a level 6 jungler would be better for my style of play.
She is simple, has pretty safe clears, and not much is required of her until level 6. I also play top and prefer being a little tank and Diana has different builds for doing that.
I think it depends on what you want to do. I prefer being able to burst people down early and later being semi-tanky so that I don't die and let the carries do their job.
I don't like becoming the carry myself when I jungle, I just feel like the gold isn't there to buy the items I need to do that.
Typically, I go
and almost always build

The boots depend on the opponents and matchup. If I am the carry I go
otherwise its
or
.
I think you should build
for her too but it really depends if you want sustained damage or burst. These items are both more for sustained damage (
). I actually run scaling CDR runes for her and think those are the best for her. I use to not run any, but I really didn't like having to build CDR items if I didn't need too simply to use my skill with greater frequency.
I kind of build for early burst and then sustain damage over time. I have actually been tinkering with the Windspeaker blessing keystone and I really like it on her. I use to go SoTA but with its removal I could go either damage or defense. I chose defense to stay healthy in the jungle.
You don't need mana items on Diana. It is better to go double or triple doran's in the early game and then rush something like a Luden's, Zhonya's, or Abyssal. Then you build as what you need for the game. She's kind of a one and done champion in the late game with the fact that she uses one to three combos in an entire fight depending on how it plays out, so mana is not a necessity in the late game when you have a large enough mana pool already. And you get more doran's in the early to make up for the fact that you have high costs but low pool. Pretty much her playstyle is to cs and land your q on the opponent and only go for trades when you land the q. Doing this, and hitting most of the CS it is very hard to run out of mana in lane.
Triple dorans would cost 1200 Dunno
You can get a similar effect by getting
starting buy, then pick up a
on your first back. Gives delicious amounts of sustain while giving you the AP you need.
But those builds up there are boring af. I always rush Abyssal (since who plays AD champs mid these days, right?), then into Nashor's, CDR boots or Sorc.Shoes as necessary, and rush big-ticket items, like Deathcap and Rylai's. Lich Bane is also good to get, instead of Frozen Fist.
Where is the
? Probably one of the better items for her.