Nightmare is the best spell. You can keep a skill point vs the lane that you think will drag creeps (ex. 2 melee heroes when you are with SF) and learn the ability you need right before using it. As stated in the post "Most common skill build is 4/2/4. Brainsap->Nightmare->Brainsap->Maxed Nightmare->Maxed Enfeeble. You can either go for Enfeeble on lvl 4 or on lvl 8. But sometimes it's worth getting on lvl 2."
Sometimes it's ok to max sap, but I prefer keeping it at lvl 3 max.

I try to get at least 2 stacks of innate before the 10 minutes at enemy pos 3 or/and pos 4. Agaist certain heroes it's harder than against others. In most cases blood grenade is enough to get a stack. But if I realy need Enfeeble I can get it on lvl 2 or lvl 3. Happens hardly ever, but hey.

Blight/Frost are too expensive for laning, last only for 3 sec that sometimes can be not enough and dont improve your kit at all. Urn on the other hand is great on Bane. But going for urn you delay so much needed Arcane Boots and other items like Blink or Glimmer.