Kroenig
Cthulhu isn't even anywhere near the most common Old One in the Lovecraft mythos... but he IS one of the most awesome. Tentacle-headed dragon man thing for the win!
Nyarlahotep is rather interesting as well.
It's true he may not be the most mentioned in the original works, but he has become rather iconic of the mythos in the time since.
Nyarlathotep "The Crawling Chaos" is rather interesting for the utter contempt he has for humanity, perhaps especially his own cultists. He is also one of the most feared as the mouthpiece of Azathoth "The Daemon Sultan" "The Blind Idiot God" the greatest and most powerful of the elder gods.