Level 1 - Victuals - Always go with this one. Great for sustain throughout the game early/mid/late stages.

Level 4 - Envenom - As with most assassins envenom is a great choice to finish off that escaping enemy.

Level 7 - Abattoir - Extra 10% bonus damage and 10% attack speed (with Blood Frenzy)!? Hell yes!

Level 10 - Lamb to the Slaughter - Great for peeling enemies or anyone who has pushed too far or away from the group. Reminds me of Rupture but in this case the enemy is forced to stay in one spot.

Level 13 - Crave Flesh - 30% move speed increase coupled with the hamstring (Q), good luck getting away from Butcher.
2nd option - Savage Charge - I would only go with this ability if they have really tanky heroes on their side. A quick 15% HP gone is pretty nice.

Level 16 - Blood Frenzy - Similar to Blood Frenzy but at level 16 and 35% instead of 20% assuming you went with Abattoir.

Level 20 - Nexus Blades/ Bolt of the Storm - This one is kind of a tossup. If your opposing team a lot of CC (roots/slows/stuns)I would go with Bolt. If not and you have some good support, definitely get Nexus Blades as it increases your damage output and another slow doesn't hurt!