usually i do this:
if i am against a high ranged champion i go for abyssal,
if he is a low ranged champ i take morello and
if he is a non ranged champ (that usually means ad) I go for zhonya's
against high ranged abyssal is the best 'cause you need magic res (he will poke you down really fast without Mres), you don't need mana reg 'cause he has better range... and than you can't spam Q to poke him. You will need otherwise a good amount of dmg and penetration so that your Q (when you can cast it safely) will do a lot of dmg. Once 6 abyssal will be also a great choice to 100-0 them.
against low ranged morello is the best 'cause you need AP (so that Q will do more dmg), CD so that you can spam Q much more, and mana reg so that you can continue to spam Q and poke him down. Pratically against low ranged you just need to continually poke him down with Q and you can do it easily since he is low ranged. Once low enough and 6 you just got a kill.
against non ranged zhonya is better just because of armor. In addition you will have enough AP to trade with opponent ...just be sure to go in once you already did 1,2 AA against minions.
here it is what i suggest you after playing 100 games with Diana
oh another thing is that most of the times is better to start with crystalline flask (especially against non ranged)