Who do you did you think was the cooler reaper?
I say it was easily Sovereign. Sovereign was cunning, pragmatic, and menacing. He managed to indoctrinated one of the galaxy's best spectres, took control of an entire geth faction, and nearly took control of the Citadel. He also had some of the most badass and chilling speeches ever.
Harbinger on the other hand was a fucking annoying troll. His lines and speeches were so comical they become popular internet memes. His actions, plans, and motivations were contrived compared to Sovereigns. He's a joke among the face of reapers, and the conversation you have with him in Arrival was lame compared to Sovereign's on Virmire. Sovereign wins, hands down IMO.
BTW, this is coming from someone who likes Mass Effect 2 more than the original Mass Effect. :)