Illidan. A Melee Assassin who can be a nightmare for teams focused on Basic Attacks, but is unusually vulnerable to burst Spell damage, Blinds, Roots, and Healing Received penalties. Very versatile, able to take any Mercenary camp easily at essentially any stage of the game, able to put surprising pressure on unattended Structures, execute daring dives into enemy territory to finish frail foes... but also has a very high skill floor, where it's easy for even a fairly experienced player to misjudge and die having accomplished nothing.
Murky. A 'joke character' Melee Assassin with incredible waveclear, terrifying dueling capacity in the late game with the right Talents, and whose death is usually inconsequential, allowing him to freely do things that would be unacceptably suicidal on many other Heroes, such as dive deep into enemy territory in an obvious manner.
The Butcher. A Melee Assassin who starts off with poor damage and limited utility, but if he can keep being present for the deaths of enemy Minions or especially enemy Heroes, he can grow into an unkillable monster that solo-kills anyone in the game... so long as Blinds and Roots don't get in his way.
Zeratul. An extremely mobile Stealthed Melee Assassin, with high burst damage, incredible potential for pursuits and escapes, significant mindgame ability, and can Talent to become a nearly unkillable duelist.
Alarak. A Melee Assassin who can pull enemies out of position, shut off their Abilities, and output a lot of damage very suddenly, but who has a notable element of 'momentum', where a well-played Alarak is incredibly strong until he stumbles and dies, at which point he loses a good chunk of his power and has to work to re-earn it.
Kerrigan. A Melee Assassin relying on an unfortunately-rigid combo, who performs well against teams with low damage throughput so her Assimilation Shields can continuously prevent damage from reaching her actual HP.
Samuro. A Stealthy Melee Assassin who leaves enemies perpetually unsure of his true location, rarely needs serious breaks from the fight if played well, and can execute unwary roamers with minimal risk of backfire. If played carelessly, potentially feeds the enemy team lots of progress on Quests that require hitting Heroic targets.
Maiev. A mobile Melee Assassin good at forcing enemies to stay still, pulling enemies where they don't want to be, and punishing clumping of any kind, with excellent ability to neuter highly telegraphed attacks aimed at her, but absolutely no self-sustain.
Valeera. A Stealth-focused Melee Assassin who has significant ability to burst out a lot of damage, can neutralize Basic Attack-focused Heroes for an extended period, and in general is focused on ruining Heroes. Has good base bulk that both Heroics significantly enhance, but notably lacks HP restoration.
Qhira. A Melee Assassin with shocking ability to heal herself, excellent ability to evade attacks and escape bad situations, and very good damage output. Can sometimes struggle with getting close enough to deliver damage.