I think Gromm is the most badass of them all. Even though he betrays Thrall by accident, he was still very powerful, and if he didn't give his life to defeat Mannoroth, then maybe he would be the greatest Orc of all time.
WTF what about nerz'hul? he started the scourge dood! and now him merged with arthas is rather interesting, it have been said "he is the most powerful entitity the world has known". well yeah, the endless mental power of the lich + the strong arthas swordman that pwned that illidan = wow.
then it would be mannoroth. hell, his hairs are always blazing! too bad he died to that orc.
but, let'S go nerzhul! rebuild your citadel or sumthing!
Illidan, of course! he's the only hero that has a voice that chills down your spine with all the hatred he has.. and he can't succeed with anything, he reminds me of myself.
Cenarius... cause hes a messed up half elf half stag freak. And hes practically impossible to kill. Or, Archimonde, cause he turns his enemies inside out.