Bloodseeker + Viper

Why this works

Picking these two together is a bet on single-target aggression. Viper’s constant slows and damage-over-time from Poison Attack make an enemy an easy target. This perfectly sets up Bloodseeker's Rupture, forcing the victim to either stand still for Viper or run and take massive damage. All the while, Viper's poison keeps the enemy's health low, giving Bloodseeker the Thirst bonuses he needs to chase and finish the job. It's a simple, direct plan for getting kills.

The problem is that this plan can be too simple. The combo is focused on one hero at a time and doesn't bring much hard lockdown like stuns. In a chaotic teamfight, enemies can use saves like a Glimmer Cape or a Force Staff to negate the whole gank. If the enemy team starts grouping up early, it becomes much harder for Viper and Bloodseeker to find the isolated pickoffs they need to get ahead. They need to snowball during the mid-game, because if they don't, teams with better five-man fight can just wait them out.