I play a lot of Diana and she seems to be a good champ to carry with. She snowballs HARD and can roam pretty easily. Obviously, some people will say Zed or Ahri, since they are strong right now and fotm. But, Diana remains un-nerfed and is very fun to play. Give her a shot.
In mid lane, my first item is almost always either abyssal or zhonyas depending on who I'm laning against. When you hit 6, you land a combo and it's pretty much over from that point if you have been laning effectively.
My sequence for combo is, W,Q,R,AA,E,AA,R again and Q when it comes off cd. Ignite in there somewhere to guarantee the kill.
For leveling abilities, I alternate Q and W until 5 and take E. Then I level R at 6. Then I proceed to level Q, W, and R whenever it's up. E is leveled last in MOST cases.
Good luck bruh.
Edit: Diana does not fall off very hard and throughout the game she always able to one-shot the adc, supp, or any other squishy. In the current meta, with tanks and adcs, she does very well.