IF you want a really good item for tankiness and damage here is a good build

Okay Roa is awful takes to long to stack adn you need the burst at 6 so you can actually be dominant in lane. let me give you a few reasons on these items and why to chose them
So Abyssal gives mr, and ap (the rework will give health and decrease MR over time) Great stats and great passive to chuck down squishies while giving decent damage and tankiness
Rylias Partly because it will let aoe spells slow 40% in this upcoming patch gives 100 ap and will give 400 health it will be pretty decent
Rabadons will give 35% AP this next patch as some items will be decreasing in AP it may seem more assassination but will help with her overall damage you will get with Nashors
Nashors so you can auto more in close fights and proc your passive more plus it gives CDR for you shield and E
Frozen Heart gives you Armor and CDR and Mana which diana craves (usually burst Diana needs more mana though even though it doesnt fit in her build) So this is a great option to go with vs a heavy AD team (ones with Jinx, or Tryn top, or others)
I stated mercs to reduce hard cc so you can keep up in fights and is great vs APs but trust me sorcs is good too so don't cross those out as a good option
Zhonyas is pretty good too it gives a ton of armor + frozen heart is well over 240 armor :D plus the active is amazing and the AP is quite good itself