I'd have to go with the Dominion. Yes, all they'd do is kill you rather than turn you into a cybernetic zombie, but as previously stated they set their sights on conquering the AQ and set about it, throwing massive amounts of resources into it.
They've got soldiers who are ready to fight in only a couple of days, sleazy middlemen who try to worm their way in, and then leaders who can be anyone of anything. They were not above lying to their allies, turning the Cardassians into second class citozens on their own world before settling on a path of genocide. The bulk of their ships are small, fast and strong, and not above suicide runs on larger targets, whilst their motherships have some serious firepower behind them.
I might have gone with the Borg if they'd learnt from their previous error. Their first attempt on Earth involved one Cube. Their second attempt on Earth involved one Cube. Then after the Queen abducted Seven, she proposed using an assimilation virus on humanity. Why not simply dispatch 1000 Cubes? There is no way in hell Starfleet could ever beat that, then one Cube could be dispatched to each 150+ member worlds and begin their assimilation, whilst the others expand into Klingon, Romulan, etc territories.