Cassiopeia

Dat cass here's how I do it, solo mid or top is pretty much necessary.

Runes
Reds - Magic Pen
Yellows - mp5/lvl
Blues - cdr/lvl
Either HP, Magic Pen, or AP quints. I usually do 2 HPs and an AP for a touch of early rape.
Blitz pointed out ap/lvl Blues, these are also effective though her ult is wat makes her useful so cdr is wat I usually roll.

Masteries
9/0/21
Standard Mage

Summoner Spells
Ghost, Flash
I've never even considered anything else bro. You can use flash to pop into an enemy's sight and ult their face, ghost also helps proper placement and escaping. Remember that successful Qs will speed cass up, so when you're running try to land one to get a boost as well.

Skills
At level 1 get either your Q or W, I usually go with W because it's easier to land. Get both of those and a point in E by level 3. If your opponent is slow, level Q first, but if you cant hit him or there are two, level W. After 6 level E then W and finally Q. Your W and Q will become reactive, it depends on how successful you are at landing your Q on champs.

Items

1. Doran's Ring
   
2. Ruby Crystal and Amp tome (ONLY IF MID/TOP, if laning go with basic boots first. I skip them until after a guise is done in mid, her q speeds you up bro)
   
3. Haunting Guise, Sorc Boots
   
4. Rylais
   
5. Abyssal Scepter for some more pen and MR, Zhonyas if you need armor
   
6. Wildcard reactive AP items depending on opposition's tank and watnot