I take ignite in most match ups mid. Use it when you go in for a kill to reduce their healing and deal that little extra true damage. I often see mid laners use it after their combo as a finisher, I'm pretty sure that's dumb. Why just do the damage when you can do the damage and reduce healing from potions, life steal and passive Regen during the fight as well.
Teleport in mid is great if you don't think you're gonna be able to get solo kills on your lane opponent. i take it against long ranged poke heavy opponents such as Xerath Ziggs Azir Oriana Vel'Koz Cassiopia. Use it to get back to lane early game so that you dont fall too far behind. late game use it to split push and tp back for teamfights.
If you want to take Diana top take teleport. She can top but it isn't strong in my opinion.
If you're jungling. Take Smite. If you don't, you're dumb.
I guess you could take cleanse on Diana mid. i wouldn't, but I'm bronze so y'know, do whatever you want.
I take this mastery page on Diana mid. I'll explain each meaningful choice here.
Fury: Attack speed is good on Diana as it helps you to proc your passive (Moonsilver Blade) more. Sorcery is good too but i personally feel the 2% increase to my ap to be insignificant compared to the attack speed increase.
Double Edged Sword: More damage helps with killing people. seems weird right? the downside is worth it in my opinion. Feast is good if you're a wuss as it helps to sustain you through tough matchups. i do take it against poke heavy champs.
Vampirism: Natural talent sucks so i take this instead. It also helps make up for the extra damage you take because of Double Edged Sword.
Oppressor: Your E ( Moonfall) inflicts a minor slow onto your opponent. Because i incorporate the spell into my combo oppressor gives me more damage. YAY MORE DAMAGE :D
Wanderer: Helps get you places faster. Take savagery if you're having trouble last hitting.
Secret Stash: getting a little extra out of your potions helps with laning a lot. the 15 instant mana you get on use sounds like nothing but has actually helped me get barely enough mana for the last q to secure kills on numerous occasions.
Meditation: The mana regen helps a lot with the early game. I use mana like candy cause I'm bad so this mastery helps mitigate that a little.
Dangerous Game: This saves my life so much its not funny. You're not a support. Don't take bandit.
Intelligence: Precision is probably better but i prefer the cooldown reduction personally. do what you want the benefits are minuscule.
Thunderlords Decree: This helps with your burst and with level 1 trades as your W (Pale Cascade) will proc TLD on its own
Runic Mastery: Biscuits are good but having your buffs for longer as a jungler is so much better.
Stormraiders Surge: I take this on Jungle Diana every time as Thunderlords Decree is good for early trades in lane. In the jungle, you aren't taking early trades in lane. Stormraiders Surge helps you to stick onto your target when you go in for ganks and also helps you to escape in teamfights and skirmishes after you take your primary target down.
Strength of the Ages: Obviously this would change your entire mastery page, but you're smart enough to do that yourself. It's good if you want to be more beefy and it gives you awesome sustain once you have the bonus 300 hp. take it if you want.
Passive (Moonsilver Blade): This is an amazing Auto Attack buff. it does more damage than your ultimate. Seriously, It hits like a truck. It flattens towers with ease and is an excellent source of sustained damage in fights and skirmishes. It does make it hard at times to freeze lane so be careful with that. Before you go in for a trade it pays to auto attack a minion or monster twice so you can incorporate a cleave into your combo.
Q (Crescent Strike): This is Diana's main damage ability. It does decent damage at range and the moonlight debuff is the most important thing in Diana's entire kit. It's hitbox is kinda weird to get used to at first but you get used to it pretty quickly. Remember that it always curves around from Diana's Right Side, which is the side she holds her weapon.
W (Pale Cascade): Pale Cascade is the ability that makes jumping into the middle of a 5 man team on your own look like a good idea (its not, by the way). The defensive power of this ability at rank 5 is disgusting. It's the ability that allows Diana to farm the Jungle and the best way to win early trades with its ability to proc Thunderlords Decree on it's own. When you go in with your R if there is no minions or tanks between you and that juicy 8/0/3 Jinx press your W as you press R. This allows you to almost instantly hit the target with all 3 spheres rather than having to wait for them to spin around you and hit one at a time.
E (Moonfall): Diana's E is an ok ability. Use it to stop your opponents from escaping you and to disrupt teamfights. Aside from that, it deals no damage and the cc is minor. It also helps with escaping, it seems counter intuitive to pull that 12 and 0 tryndamere towards you when you're running from him, but the slow your E provides will gain you distance in the long run trust me.
Skill Order (lane)
Take W level 1 as it helps with trades and lets you take CS without taking too much punishment from aggressive opponents. It also procs Thunderlords Decree on it's own if you land all 3 spheres on your opponent.
you can take your E at level 3 if your jungler wants to gank mid early. Don't put the point in until you're about to throw out your 1st q after you hit 3.
Skill Order (jungle)
Take a second point in your W at level 3 then continue as normal taking E at level 4 and maxing your Q, then W Then E and R at levels 6,11 and 16.