My recommendation for a strong MF.

Buy these runes. Gives you good armor penetration to hurt the enemy early. Also gives a tonne of mana regen and some early life.

Purchase in order of this priority list:
1. Quints of Fortitude
2. Seals of Clarity
3. Marks of Desolation
4. Greater Quint of Desolation
5. Glyphs of Clarity

These are good masteries for MF.

Here would be my order:
1/1 Haste
1/1 Cripple
3/3 Perserverance
4/4 Awareness
3/3 Deadliness
4/4 Sorcery
1/1 Archaic Knowledge
3/3 Sunder
3/3 Brute Force
4/4 Alacrity
1/3 Lethality
1/1 Havoc

Ghost is great for chasing, escaping, or getting back to your lane/team.

Exhaust is perfect for ganking, getting away from someone, or weakening the opposing teams strongest player in a team fight.

This sequence prioritizes MFs strongest skills first.

MF gets the most benefit from attack damage as it powers her auto-attack, double up, and her ultimate.

Doran's shield gives good defense, 120hp, and some great health regen as you stay back and snipe minions early. I find this item gives great advantage in 1v1. Sell this late game for an extra slot and a couple hunderd gold. Alternatively you could start with Boots and 3x HP Pots if you prefer.

After that build your Berzerkers Greaves and Bloodthirster. This can take some time but it's worth it. Then build your Zeal which will boost your attack speed.

Then it's time for a defensive item. Banshee's Veil is the best for protection but if you aren't dying much then a Frozen Mallet gives lots of health, more damage and a great slow on the enemies. In the case of Frozen Mallet, build the Phage first and then finish it up.

Now turn your Zeal into a Phantom Dancer and move on to building a Last Whisper which gives a tonne more damage through the armor reduction. Hopefully you've won by now but if not think about building a Madred's or maybe another defensive item (like Frozen Mallet or Banshee's Veil ... whichever one you didn't build earlier).