I don't think Diana is in a good competitive spot. If you're playing normal's, or low elo, you can master her and get pretty good, and carry. But her winrate in mid lane and jungle are both sub-par.
On the upside , she clears at a healthy HP level and fairly quick, if you take early points in W. She has the kit to assassinate low hp, and squishy targets with ease. And can easily solo Dragons.
On the downside, her mobility is heavily reliant on her ability to land Q's. Since she is coming out of the jungle role it may require you to be one of the primary engage roles. Which will cause you to get focused and bursted down when you jump in.
Her Kit does however allow her to be vastly versatile in her build paths
The most optimal build acording to Champions.gg Is : Enchantment Runic Echos (Red Smite) , sorc boots, Nashors, Lich Bane, Void Staff, Liandrys
What I Typically prefer in the jungle : Enchantment Runic Echos (Blue Smite), Sorc Boots, Zhonya, Lich Bane, Voidstaff, Rylais(deatch cap if ahead) * i like her kit to be more sticky (Blue Smite, Rylias) to track down champs, accommodate for missing a Q, or opponents flashing.
Other items she can substitute :
If needs more durability can effectively use Ice Born Gauntlet instead of Liche Bane
My Preferred Jungle Rune path is :
Domination : Electrocute , Sudden Impact, Eyeball Collection, Relentless Hunter
Sorcery : Absolute Focus, Gathering Storm
**Precision : Triumph, Coupe De Grace or Legend : Alacrity
(Sorcery can be substituted for Precision)