Invoker.
The way his voice echoes combined with his voice-lines makes it feel you're playing a Sith Lord or something. That along with the fact that he is so versatile and can achieve so much if played to his full potential makes him a hero you always feel like you're improving with as you play, even if you are losing.
Finally, Invoker is a smug motherfucker, which makes it even more hilarious when you kill people.
Other than Invoker, Kunkka, Shadow Demon, Dazzle, Ancient Apparition, Juggernaut and Omniknight are also up there
OG LOOK AT IT GO