I find ETC to be fantastic. Tanky as a mofo, he's got good mobility with his powerslide able to slide in to engage and stun, or slide out if he needs to disengage. Face melt combos great with this, powerslide onto the opposite side of someone, wait a moment to get the full effect out of the stun, and then facemelt them backwards into your team. I've found that abilities that move the enemy are really powerful. And then you've got guitar solo, which not only keeps him healed up, but with his passive allows you to spam it and keep your team buffed or buff minions to push down lanes even better. And both of his ultimates are great, with one providing stuns for days and the other allowing you to split push a lane and then teleport to your team should a teamfight break out.
My favorite assassin character is by far Tychus. He's got the minigun putting out some hurtful aoe, he's got the grenade that can be built to decimate minions/monsters/structures and even without talents has great pushback (again, anything to move the enemy) and with the right build he can stay in the fight a long time. First aid to heal up, commandeer odin for an extra lifebar, he even gets access to stoneskin, which is usually only for tanks.