That's a hard question to answer. Both the Cybermen and the Daleks are classic Doctor Who enemies and Doctor Who would not be the same without them.
Both the Cybermen and the Daleks are equally terrifying villains in the fact that neither are capable of feeling emotion (with the exception of a few Daleks) and they kill without mercy or remorse. Both species desire to eradicate all lifeforms that are not one of them.
As awesome as the Cybermen are, I will cast my vote for Daleks.
The Daleks kidnapped the Earth and several other planets in perhaps one of the most elaborate plans for destroying the Doctor and his friends. The Cybermen have had some crazy plans, but none so elaborate as to move planets across the universe.
Then again, the Cybermen did turn the Doctor into a partial Cybermen in Matt Smith's last season.
Ah, never mind it's too hard to choose one!